This is helpful for upcoming testing work which allows datadriven
testing of the interaction of multiple nodes. This testing requires
determinism to work correctly.
This change introduces joint quorums by changing the Node and RawNode
API to accept pb.ConfChangeV2 (on top of pb.ConfChange).
pb.ConfChange continues to work as today: it allows carrying out a
single configuration change. A pb.ConfChange proposal gets added to
the Raft log as such and is thus also observed by the app during Ready
handling, and fed back to ApplyConfChange.
ConfChangeV2 allows joint configuration changes but will continue to
carry out configuration changes in "one phase" (i.e. without ever
entering a joint config) when this is possible.
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.
