transactions: Use shared Vote type across Praefect and Gitaly

Praefect and Gitaly are currently using two separate implementations of votes. This MR addresses this by moving Gitaly's Vote type into a central shared package internal/transactions/voting and converts Praefect to use it, too.

This is only the first step -- I think we should also be moving the internal/praefect/metadata package into internal/transactions/txinfo or similar given that it's shared across both implementations and that the package name is a bit lacking.