etcdserver: use PreVote so majority partition's leader isn't forced to step down

[heyitsanthony]

If a cluster is partitioned then connectivity is fixed, the minority partition can trigger a new election that will cause the majority partition's leader to step down and disrupt availability. The fix, as described in 9.6 of the raft disseration, is PreVote.

[BlueBlue-Lee]

I have found the implementation in etcd. Do I miss important point?

[irfansharif]

@shenlanse: it isn't as stable/battle tested as yet, see #8501, #8498 and the issues linked therein. We (cockroachdb/cockroach) have run into issues enabling it as it stands today.

[BlueBlue-Lee]

@irfansharif Ohh，thanks, I will see it

[gyuho]

Will be enabled by default in v3.5. Has been testing pre-vote in functional testing for several months, and seems stable.