This is helpful to quickly print the configuration log messages without
having to specify Voters and Learners separately.
It will also come in handy for joint quorums because it allows holding
on to voters and learners as a unit, which is useful for unit testing.
The quorum package contains logic to reason about committed indexes as
well as vote outcomes for both majority and joint quorums. The package
is oblivious to the existence of learner replicas.
The plan is to hook this up to etcd/raft in subsequent commits.
