Adding ZMQ/Pub support for txpool_add and chain_main events

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+# The Current/Future Status of ZMQ in Monero
+
+## ZMQ Pub/Sub
+Client `ZMQ_SUB` sockets must "subscribe" to topics before it receives any data.
+This allows filtering on the server side, so network traffic is reduced. Monero
+allows for filtering on: (1) format, (2) context, and (3) event.
+
+ * **format** refers to the _wire_ format (i.e. JSON) used to send event
+   information.
+ * **context** allows for a reduction in fields for the event, so the
+   daemon doesn't waste cycles serializing fields that get ignored.
+ * **event** refers to status changes occurring within the daemon (i.e. new
+   block to main chain).
+
+ * Formats:
+   * `json`
+ * Contexts:
+   * `full` - the entire block or transaction is transmitted (the hash can be
+     computed remotely).
+   * `minimal` - the bare minimum for a remote client to react to an event is
+     sent.
+ * Events:
+   * `chain_main` - changes to the primary/main blockchain.
+   * `txpool_add` - new _publicly visible_ transactions in the mempool.
+     Includes previously unseen transactions in a block but _not_ the
+     `miner_tx`. Does not "re-publish" after a reorg. Includes `do_not_relay`
+     transactions.
+
+The subscription topics are formatted as `format-context-event`, with prefix
+matching supported by both Monero and ZMQ. The `format`, `context` and `event`
+will _never_ have hyphens or colons in their name. For example, subscribing to
+`json-minimal-chain_main` will send minimal information in JSON when changes
+to the main/primary blockchain occur. Whereas, subscribing to `json-minimal`
+will send minimal information in JSON on all available events supported by the
+daemon.
+
+The Monero daemon will ensure that events prefixed by `chain` will be sent in
+"chain-order" - the `prev_id` (hash) field will _always_ refer to a previous
+block. On rollbacks/reorgs, the event will reference an earlier block in the
+chain instead of the last block. The Monero daemon also ensures that
+`txpool_add` events are sent before `chain_*` events - the `chain_*` messages
+will only serialize miner transactions since the other transactions were
+previously published via `txpool_add`. This prevents transactions from being
+serialized twice, even when the transaction was first observed in a block.
+
+ZMQ Pub/Sub will drop messages if the network is congested, so the above rules
+for send order are used for detecting lost messages. A missing gap in `height`
+or `prev_id` for `chain_*` events indicates a lost pub message. Missing
+`txpool_add` messages can only be detected at the next `chain_` message.
+
+Since blockchain events can be dropped, clients will likely want to have a
+timeout against `chain_main` events. The `GetLastBlockHeader` RPC is useful
+for checking the current chain state. Dropped messages should be rare in most
+conditions.
+
+The Monero daemon will send a `txpool_add` pub exactly once for each
+transaction, even after a reorg or restarts. Clients should use the
+`GetTransactionPool` after a reorg to get all transactions that have been put
+back into the tx pool or been invalidated due to a double-spend.
+
+