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

I guess this explains it:

"Next is consistency. If some of the methods of the type must have
pointer receivers, the rest should too, so the method set is consistent
regardless of how the type is used." --
https://golang.org/doc/faq#methods_on_values_or_pointers

In this case, though, they can all have value receivers, so let's go
with that for now.