Flexible quorums for even sized clusters

[kjnilsson]

Flexible quorum is the observation that as long as the election quorum and the commit quorum intersect data safety can be maintained.

For odd sized clusters this has little benefit in practice but for even sized quorums it could be beneficial in reducing the negative aspect of even sized clusters (commit latency). As can be seen in the table below the commit quorum for a 6 node cluster can be kept the same as for a 5 node cluster (3) as long as the election quorum is still a "simple quorum", i.e. 4.

Cluster Size	Election Quorum	Commit Quorum
1	1	1
2	2	1
3	2	2
4	3	2
5	3	3
6	4	3
7	4	4

[michaelklishin]

We can leave a link to FlexiRaft here FTR.