raft: recover from situations where majority of replicas are lost

[bharathv]

Support unclean leader election from a minority replica. This is useful when majority of the replicas are gone (eg: lost nodes) but a user intends to promote (one of) the minority replicas into a leader and up-replicate from there.

The solution needs to handle fencing of lost nodes if they come back up.

[rystsov]

We should make sure that the recovered partition (with a potential data loss) won't block the tx coordinator if it was involved in an ongoing transaction - #9330

[rystsov]

We already have some API for that, probably we should add a script to list the transactions on the coordinator, then check the partitions / consumer groups, find the ones which don't have the matching ongoing txes and exclude them from the coordinator.

[bpraseed]

@mattschumpert - this is flagged from chaos testing.
https://docs.google.com/document/d/110nU8vDI8YpQ25IHvNlEuTTK69zplzfkzWeA-TH4LyE/edit#heading=h.yxgertgn7rj8