Feature: withdraw to multiple addresses

[ZmnSCPxj]

Because batching is a good thing, especially if we want to support c*stodial services that might have multiple withdrawal requests from its victims.

It is also a good preparation for eventually implementing #1936 , which requires building transactions that pay to multiple outputs, by combining a multi-output withdraw with #2672 (by using our own wallet as the "external" wallet).

[trueptolemy]

@ZmnSCPxj The present format is: *txprepare* destination satoshi [feerate] [minconf].
What about letting txprepare also support another format:
*txprepare* {{'destination': destination1, 'satoshi': amount1}, {'destination': destination2, 'satoshi': amount2}, {'destination': destination3, 'satoshi': amount3}} [feerate] [minconf]

[ZmnSCPxj]

This makes help for txprepare rather ambiguous: which format do we inform?

It might be better to switch txprepare to the latter format and support the existing format for back-compatibility until we remove it in a later release, a la #2914

[ZmnSCPxj]

While txprepare now allows multiple outputs, there is no convenient user command for withdrawing to multiple addresses yet.