[REVIEW REQUEST] KAFKA-3184: Add Checkpoint for In-memory State Store

[Nikolay Izhikov]

Hello, Kafka Team.

I prepared a PR [1] for the KAFKA-3184 [2]
Can someone, please, do the review.


[1] https://github.com/apache/kafka/pull/8592
[2] https://issues.apache.org/jira/browse/KAFKA-3184