Voting Pools - Deposit side

This adds a new votingpool package containing the basic data structures to implement voting pools and DepositScriptAddress(), which generates a multi-sig script for the given series/branch/index and returns its P2SH address.

As there's no generic API to access the wallet's DB (yet), some of our code has to live in the waddrmgr package (in the votingpool_bridge.go file) so that it has access to the DB API.

(I'm the one submitting this, but this is really a joint effort from myself, @larshesel, @teknico and @jimmysong, so any praise/blame should be shared between all of us)
A minor optimization would be to calculate the full size of the serialized bytes up front and do `serialized := make([]byte, 0, serializedLen)` so append doesn't have to grow the backing array and copy multiple times.