Add support for external state (log) to influence leader voting. This,
in effect, implements lastLogIndex and lastLogTerm sent on RequestVote
RPCs from the Raft paper. This works by exposing two callbacks: one that
calls into the user on RequestVote to get the candidate's state and one
that calls into the user upon receiving a RequestVote to determine if a
vote should be granted based on comparing the logs.

From Raft:
1. Reply false if term < currentTerm (§5.1)
2. If votedFor is null or candidateId, and candidate’s log is at
least as up-to-date as receiver’s log, grant vote (§5.2, §5.4)