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Leader Progress Timeout

In the traditional push-based model, when a leader is disconnected from the quorum due to network partition, it will start a new election to learn the active quorum or form a new one immediately. In the pull-based model, however, say a new leader has been elected with a new epoch and everyone has learned about it except the old leader (e.g. that leader was not in the voters anymore and hence not receiving the BeginQuorumEpoch as well), then that old leader would not be notified by anyone about the new leader / epoch and become a pure "zombie leader", as there is no regular heartbeats being pushed from leader to the follower. This could lead to stale information being served to the observers and clients inside the cluster.

To resolve this issue, we will piggy-back on the "quorum.fetch.timeout.ms" config, such that if the leader did not receive Fetch requests from a majority of the quorum for that amount of time, it would begin a new election and start sending VoteRequest to voter nodes in the cluster to understand the latest quorum. If it couldn't connect to any known voter, the old leader shall keep starting new elections and bump the epoch. And if the returned response includes a newer epoch leader, this zombie leader would step down and becomes a follower. Note that the node will remain a candidate until it finds that it has been supplanted by another voter, or win the election eventually.

As we know from the Raft literature, this approach could generate disruptive voters when network partitions happen on the leader. The partitioned leader will keep increasing its epoch, and when it eventually reconnects to the quorum, it could win the election with a very large epoch number, thus reducing the quorum availability due to extra restoration time. Considering this scenario is rare, we would like to address it in a follow-up KIP.

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Note that followers will have to check any records received for the presence of control records. Specifically a follower/observer must check for voter assignment messages which could change its role.

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There will be two options available with -- describe : 

• describe --describe status: a short summary of the quorum status and the other provides detailed information about the status of replication.
• describe --describe replication: provides detailed information about the status of replication

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> bin/kafka-metadata-quorum.sh describe --describestatus
ClusterId:              SomeClusterId
LeaderId:				0
LeaderEpoch: 			15
HighWatermark:			234130
MaxFollowerLag: 		34
MaxFollowerLagTimeMs:	15
CurrentVoters:			[0, 1, 2]

> bin/kafka-metadata-quorum.sh describe --describe replication
ReplicaId	LogEndOffset	Lag		LagTimeMs	Status
0			234134			0		0			Leader
1			234130			4		10			Follower
2			234100			34		15			Follower
3			234124			10		12			Observer
4			234130			4		15			Observer

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