daemon, wallet: new pay for RPC use system

Daemons intended for public use can be set up to require payment
in the form of hashes in exchange for RPC service. This enables
public daemons to receive payment for their work over a large
number of calls. This system behaves similarly to a pool, so
payment takes the form of valid blocks every so often, yielding
a large one off payment, rather than constant micropayments.

This system can also be used by third parties as a "paywall"
layer, where users of a service can pay for use by mining Monero
to the service provider's address. An example of this for web
site access is Primo, a Monero mining based website "paywall":
https://github.com/selene-kovri/primo

This has some advantages:
 - incentive to run a node providing RPC services, thereby promoting the availability of third party nodes for those who can't run their own
 - incentive to run your own node instead of using a third party's, thereby promoting decentralization
 - decentralized: payment is done between a client and server, with no third party needed
 - private: since the system is "pay as you go", you don't need to identify yourself to claim a long lived balance
 - no payment occurs on the blockchain, so there is no extra transactional load
 - one may mine with a beefy server, and use those credits from a phone, by reusing the client ID (at the cost of some privacy)
 - no barrier to entry: anyone may run a RPC node, and your expected revenue depends on how much work you do
 - Sybil resistant: if you run 1000 idle RPC nodes, you don't magically get more revenue
 - no large credit balance maintained on servers, so they have no incentive to exit scam
 - you can use any/many node(s), since there's little cost in switching servers
 - market based prices: competition between servers to lower costs
 - incentive for a distributed third party node system: if some public nodes are overused/slow, traffic can move to others
 - increases network security
 - helps counteract mining pools' share of the network hash rate
 - zero incentive for a payer to "double spend" since a reorg does not give any money back to the miner

And some disadvantages:
 - low power clients will have difficulty mining (but one can optionally mine in advance and/or with a faster machine)
 - payment is "random", so a server might go a long time without a block before getting one
 - a public node's overall expected payment may be small

Public nodes are expected to compete to find a suitable level for
cost of service.

The daemon can be set up this way to require payment for RPC services:

  monerod --rpc-payment-address 4xxxxxx \
    --rpc-payment-credits 250 --rpc-payment-difficulty 1000

These values are an example only.

The --rpc-payment-difficulty switch selects how hard each "share" should
be, similar to a mining pool. The higher the difficulty, the fewer
shares a client will find.
The --rpc-payment-credits switch selects how many credits are awarded
for each share a client finds.
Considering both options, clients will be awarded credits/difficulty
credits for every hash they calculate. For example, in the command line
above, 0.25 credits per hash. A client mining at 100 H/s will therefore
get an average of 25 credits per second.
For reference, in the current implementation, a credit is enough to
sync 20 blocks, so a 100 H/s client that's just starting to use Monero
and uses this daemon will be able to sync 500 blocks per second.

The wallet can be set to automatically mine if connected to a daemon
which requires payment for RPC usage. It will try to keep a balance
of 50000 credits, stopping mining when it's at this level, and starting
again as credits are spent. With the example above, a new client will
mine this much credits in about half an hour, and this target is enough
to sync 500000 blocks (currently about a third of the monero blockchain).

There are three new settings in the wallet:

 - credits-target: this is the amount of credits a wallet will try to
reach before stopping mining. The default of 0 means 50000 credits.

 - auto-mine-for-rpc-payment-threshold: this controls the minimum
credit rate which the wallet considers worth mining for. If the
daemon credits less than this ratio, the wallet will consider mining
to be not worth it. In the example above, the rate is 0.25

 - persistent-rpc-client-id: if set, this allows the wallet to reuse
a client id across runs. This means a public node can tell a wallet
that's connecting is the same as one that connected previously, but
allows a wallet to keep their credit balance from one run to the
other. Since the wallet only mines to keep a small credit balance,
this is not normally worth doing. However, someone may want to mine
on a fast server, and use that credit balance on a low power device
such as a phone. If left unset, a new client ID is generated at
each wallet start, for privacy reasons.

To mine and use a credit balance on two different devices, you can
use the --rpc-client-secret-key switch. A wallet's client secret key
can be found using the new rpc_payments command in the wallet.
Note: anyone knowing your RPC client secret key is able to use your
credit balance.

The wallet has a few new commands too:

 - start_mining_for_rpc: start mining to acquire more credits,
regardless of the auto mining settings
 - stop_mining_for_rpc: stop mining to acquire more credits
 - rpc_payments: display information about current credits with
the currently selected daemon

The node has an extra command:

 - rpc_payments: display information about clients and their
balances

The node will forget about any balance for clients which have
been inactive for 6 months. Balances carry over on node restart.

4 files changed, 50 insertions, 7 deletions

diff --git a/contrib/epee/include/int-util.h b/contrib/epee/include/int-util.h
index 939c018ea..dcd58a8c4 100644
--- a/contrib/epee/include/int-util.h
+++ b/contrib/epee/include/int-util.h
@@ -132,6 +132,23 @@ static inline uint32_t div128_32(uint64_t dividend_hi, uint64_t dividend_lo, uin
 // Long divisor with 2^64 base
 void div128_64(uint64_t dividend_hi, uint64_t dividend_lo, uint64_t divisor, uint64_t* quotient_hi, uint64_t *quotient_lo, uint64_t *remainder_hi, uint64_t *remainder_lo);
 
+static inline void add64clamp(uint64_t *value, uint64_t add)
+{
+  static const uint64_t maxval = (uint64_t)-1;
+  if (*value > maxval - add)
+    *value = maxval;
+  else
+    *value += add;
+}
+
+static inline void sub64clamp(uint64_t *value, uint64_t sub)
+{
+  if (*value < sub)
+    *value = 0;
+  else
+    *value -= sub;
+}
+
 #define IDENT16(x) ((uint16_t) (x))
 #define IDENT32(x) ((uint32_t) (x))
 #define IDENT64(x) ((uint64_t) (x))
diff --git a/contrib/epee/include/math_helper.h b/contrib/epee/include/math_helper.h
index ddc1f7f4b..604a04680 100644
--- a/contrib/epee/include/math_helper.h
+++ b/contrib/epee/include/math_helper.h
@@ -259,6 +259,11 @@ namespace math_helper
         m_last_worked_time = get_time();
 		}
 
+    void trigger()
+    {
+      m_last_worked_time = 0;
+    }
+
 		template<class functor_t>
 		bool do_call(functor_t functr)
 		{
diff --git a/contrib/epee/include/serialization/keyvalue_serialization.h b/contrib/epee/include/serialization/keyvalue_serialization.h
index 5459c8409..78d294d05 100644
--- a/contrib/epee/include/serialization/keyvalue_serialization.h
+++ b/contrib/epee/include/serialization/keyvalue_serialization.h
@@ -26,6 +26,7 @@
 
 #pragma once
 
+#include <type_traits>
 #include <boost/utility/value_init.hpp>
 #include <boost/foreach.hpp>
 #include "misc_log_ex.h"
@@ -45,18 +46,20 @@ public: \
   template<class t_storage> \
   bool store( t_storage& st, typename t_storage::hsection hparent_section = nullptr) const\
   {\
-    return serialize_map<true>(*this, st, hparent_section);\
+    using type = typename std::remove_const<typename std::remove_reference<decltype(*this)>::type>::type; \
+    auto &self = const_cast<type&>(*this); \
+    return self.template serialize_map<true>(st, hparent_section); \
   }\
   template<class t_storage> \
   bool _load( t_storage& stg, typename t_storage::hsection hparent_section = nullptr)\
   {\
-    return serialize_map<false>(*this, stg, hparent_section);\
+    return serialize_map<false>(stg, hparent_section);\
   }\
   template<class t_storage> \
   bool load( t_storage& stg, typename t_storage::hsection hparent_section = nullptr)\
   {\
     try{\
-    return serialize_map<false>(*this, stg, hparent_section);\
+    return serialize_map<false>(stg, hparent_section);\
     }\
     catch(const std::exception& err) \
     { \
@@ -65,13 +68,22 @@ public: \
       return false; \
     }\
   }\
-  template<bool is_store, class this_type, class t_storage> \
-  static bool serialize_map(this_type& this_ref,  t_storage& stg, typename t_storage::hsection hparent_section) \
-  { 
+  /*template<typename T> T& this_type_resolver() { return *this; }*/ \
+  /*using this_type = std::result_of<decltype(this_type_resolver)>::type;*/ \
+  template<bool is_store, class t_storage> \
+  bool serialize_map(t_storage& stg, typename t_storage::hsection hparent_section) \
+  { \
+    decltype(*this) &this_ref = *this;
 
 #define KV_SERIALIZE_N(varialble, val_name) \
   epee::serialization::selector<is_store>::serialize(this_ref.varialble, stg, hparent_section, val_name);
 
+#define KV_SERIALIZE_PARENT(type) \
+  do { \
+    if (!((type*)this)->serialize_map<is_store, t_storage>(stg, hparent_section)) \
+      return false; \
+  } while(0);
+
   template<typename T> inline void serialize_default(const T &t, T v) { }
   template<typename T> inline void serialize_default(T &t, T v) { t = v; }
 
diff --git a/contrib/epee/include/storages/http_abstract_invoke.h b/contrib/epee/include/storages/http_abstract_invoke.h
index 18b7f10c1..78e02c93a 100644
--- a/contrib/epee/include/storages/http_abstract_invoke.h
+++ b/contrib/epee/include/storages/http_abstract_invoke.h
@@ -101,7 +101,7 @@ namespace epee
     }
 
     template<class t_request, class t_response, class t_transport>
-    bool invoke_http_json_rpc(const boost::string_ref uri, std::string method_name, const t_request& out_struct, t_response& result_struct, t_transport& transport, std::chrono::milliseconds timeout = std::chrono::seconds(15), const boost::string_ref http_method = "GET", const std::string& req_id = "0")
+    bool invoke_http_json_rpc(const boost::string_ref uri, std::string method_name, const t_request& out_struct, t_response& result_struct, epee::json_rpc::error &error_struct, t_transport& transport, std::chrono::milliseconds timeout = std::chrono::seconds(15), const boost::string_ref http_method = "GET", const std::string& req_id = "0")
     {
       epee::json_rpc::request<t_request> req_t = AUTO_VAL_INIT(req_t);
       req_t.jsonrpc = "2.0";
@@ -111,10 +111,12 @@ namespace epee
       epee::json_rpc::response<t_response, epee::json_rpc::error> resp_t = AUTO_VAL_INIT(resp_t);
       if(!epee::net_utils::invoke_http_json(uri, req_t, resp_t, transport, timeout, http_method))
       {
+        error_struct = {};
         return false;
       }
       if(resp_t.error.code || resp_t.error.message.size())
       {
+        error_struct = resp_t.error;
         LOG_ERROR("RPC call of \"" << req_t.method << "\" returned error: " << resp_t.error.code << ", message: " << resp_t.error.message);
         return false;
       }
@@ -122,6 +124,13 @@ namespace epee
       return true;
     }
 
+    template<class t_request, class t_response, class t_transport>
+    bool invoke_http_json_rpc(const boost::string_ref uri, std::string method_name, const t_request& out_struct, t_response& result_struct, t_transport& transport, std::chrono::milliseconds timeout = std::chrono::seconds(15), const boost::string_ref http_method = "GET", const std::string& req_id = "0")
+    {
+      epee::json_rpc::error error_struct;
+      return invoke_http_json_rpc(uri, method_name, out_struct, result_struct, error_struct, transport, timeout, http_method, req_id);
+    }
+
     template<class t_command, class t_transport>
     bool invoke_http_json_rpc(const boost::string_ref uri, typename t_command::request& out_struct, typename t_command::response& result_struct, t_transport& transport, std::chrono::milliseconds timeout = std::chrono::seconds(15), const boost::string_ref http_method = "GET", const std::string& req_id = "0")
     {