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
• add a focused test to ensure constructor members are set

Motivation:

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

Validation:

• maven3\current\bin\mvn.cmd -pl zookeeper-server -Dtest=QuorumPeerTestBaseTest test

[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.