ZOOKEEPER-5013: Initialize MainThread members

[seekskyworld]

Issue: https://issues.apache.org/jira/browse/ZOOKEEPER-5013

Summary:

• initialize MainThread member fields in the constructor chain that writes dynamic/static configs

Motivation:

• avoid incorrect defaults (0/null) when tests query MainThread metadata

Validation:

• not run (not requested)

[anmolnar]

Please don't create test classes for test classes, otherwise you should write a test class for your test class which tests another class. When you're ready, you should write another test class which tests the test class that tests the previous test class, and so on...

[anmolnar]

Patches should be opened against the master branch at the first place.

[anmolnar]

Please see my comments above.

[seekskyworld]

@anmolnar Thanks for the feedback. I removed the test-class-for-test-class and kept only the MainThread field initialization. I
also retargeted the PR to master and force-pushed the updated branch. The diff now only touches
QuorumPeerTestBase.java. Please let me know if you want any additional changes or validation.