TODO:

• Correct settings processing from configs (timeouts and so on)
• Fix exception handling (do not use LOGICAL_ERROR in all places)
• Make TestKeeperStorage a backend for TestKeeper (remove a bunch of duplicated code)
• Check XID and ZXID for watches, now it works well with XID = ZXID =-1. -- seems like we don't need it. https://github.com/apache/zookeeper/blob/master/zookeeper-server/src/main/java/org/apache/zookeeper/server/NettyServerCnxn.java#L163 and https://github.com/apache/zookeeper/blob/master/zookeeper-server/src/main/java/org/apache/zookeeper/ClientCnxn.java#L120-L122
• Add integration tests with kazoo (our client may be substandard)
• Some better way to respond in TestKeeperTCPHandler (wait for 10 ms doesn't seem ok)
• Control session timeout and ephemeral nodes
• Control "dead" watches

Something wrong with 01509_check_many_parallel_quorum_inserts and sometimes it reproduces locally. Possibly incorrect watch processing.

01317_no_password_in_command_line -- known flaky test.
Ok, only kazoo tests left. I think we can make TestKeeperStorage as TestKeeper backend later.

Thread sanitizer fixed in #17498

Make TestKeeperStorage a backend for TestKeeper (remove a bunch of duplicated code)
Now it's not quite easy, Not going to do it in this PR.

Integration test failure is not related to changes. Tried tests multiple times and every time everything is OK.
I'm going to merge this PR without additional refactoring and comments/documentation. I think this code will change significantly after adding the RAFT implementation.

Related #15090

How to config without zookeeper ?