During a networking partition, cluster states updates (like mapping changes or shard assignments)
are committed if a majority of the masters node received the update correctly. This means that the current master has access to enough nodes in the cluster to continue to operate correctly. When the network partition heals, the isolated nodes catch up with the current state and get the changes they couldn't receive before. However, if a second partition happens while the cluster
is still recovering from the previous one *and* the old master is put in the minority side, it may be that a new master is elected which did not yet catch up. If that happens, cluster state updates can be lost.
This commit fixed 95% of this rare problem by adding the current cluster state version to `PingResponse` and use them when deciding which master to join (and thus casting the node's vote).
Note: this doesn't fully mitigate the problem as a cluster state update which is issued concurrently with a network partition can be lost if the partition prevents the commit message (part of the two phased commit of cluster state updates) from reaching any single node in the majority side *and* the partition does allow for the master to acknowledge the change. We are working on a more comprehensive fix but that requires considerate work and is targeted at 6.0.
577dcb3237