Implement raft prevote to prevent unnecessary leader election/change.

[kikimo]

Is your feature request related to a problem? Please describe.

There is no prevote in current raft implementation, when network partition happends and recovers later, a rejoin server may disrupt the cluster by starting a new election.

Describe the solution you'd like

On the paper CONSENSUS: BRIDGING THEORY AND PRACTICE 9.6(fulll version of raft paper), there describe a way called pre-vote to avoid this situation.

Describe alternatives you've considered

Additional context
Add any other context or screenshots about the feature request here.

[critical27]

I have implemented one more than one year ago, it is almost same as current implementation. The leaderElection in our code is a pre-vote.

Will focus on more serious problem of raft in 3.0. Could pick my PR later if really necessary.