simplewallet: plug a timing leak

As reported by Tramèr et al, timing of refresh requests can be used
to see whether a password was requested (and thus at least one output
received) since this will induce a delay in subsequent calls.
To avoid this, we schedule calls at a given time instead of sleeping
for a set time (which would make delays additive).
To further avoid a scheduled call being during the time in which a
password is prompted, the actual scheduled time is now randomized.

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