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.
I've just started doubting if we know which series is the active one. In the api the series is passed as well when creating a deposit address, implying that we don't know which series is the active one. I've asked Justus to clarify this.