First pass at implementing VotingPool.Withdrawal

Still lots of TODOs/XXXs and just a single (and overly long) test, but I've
been working on this too long now; it's time to get some feedback and start
tackling the remaining things step by step.

Oh, and this depends on https://github.com/gsalgado/btcscript/tree/rename-signTxOutput, which has already been submitted (but not yet accepted) upstream.
Basically the same thing as above. In the startwithdrawal, the changeaddress explicitly defines the series to use, so maybe we should not assume that we know it or even can look up the active series. I've asked justus to clarify.