Change withdrawal.sign() to lookup the master private keys from the votingpool DB and derive the ones necessary to sign transactions

Most of the types I've seen in btcwallet seem to either have all their
methods with a pointer receiver or with a value receiver, but not a mix
of both. And there seems to be more with pointer receivers than
otherwise, so I was just trying to be consistent. My impression may be
wrong, though, and there may be actual guidelines they follow for when
to use one or the other, but I can't seem to guess what they are. I'll ask