Remove zookeeper related code

[yuzhu]

What changes are proposed in this pull request?

With 3.0 architecture, Zookeeper related leader election functionality is no longer needed. This is a first attempt to remove most of the zookeeper dependency from the code base.

Does this PR introduce any user facing changes?

Removal of zookeeper dependency and the ability to use it to elect a leader.

[yuzhu]

still fixing some tests, but welcome high level comments

[yuzhu]

@apc999 @jiacheliu3 PTAL thanks.

[cosminnicula]

Hey @yuzhu, by the looks of this PR, the code changes should clear the path toward removing the org.apache.zookeeper:zookeeper dependencies from all the pom.xml files.

If this holds true, then I have a few questions:

1. shouldn't this PR also clean up all the pom.xml files? (e.g. remove all org.apache.zookeeper:zookeeper dependencies from all the pom.xml files)
2. was there any real workload testing and validation that used a tarball published with these changes?
3. is there any chance to port these changes to the 2.x branch?

I'm asking all this in the context of the critical log4j 1.x vulnerabilities that are currently introduced by the org.apache.zookeeper:zookeeper:3.5.5 dependency. Removing ZooKeeper altogether would definitely help, alongside this PR #17002 which bumps hadoop from 3.3.1 to 3.3.4 (hadoop 3.3.1 was another source of critical log4j 1.x vulnerabilities).

I might be able to provide some help as well.