Add write REST endpoints to helix rest for metadata store directory

[huizhilu]

Issues

• My PR addresses the following Helix issues and references them in the PR description:

Implements #756

Description

• Here are some details about my PR, including screenshots of any UI changes:

We have metadata store directory service to help scale out zookeeper. Metadata store directory service provides REST APIs to access.
This PR the adds MSDS write endpoints to Helix REST.

Tests

• The following tests are written for this issue:

• testAddMetadataStoreRealm

• testDeleteMetadataStoreRealm

• testAddShardingKey

• testDeleteShardingKey

• The following is the result of the "mvn test" command on the appropriate module:

Tests depend on the write operations in ZkMeatadataStoreDirectory. Needs to enable asserts before tests are really testing the operations successfully.

[INFO] Tests run: 140, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 26.924 s - in TestSuite
[INFO]
[INFO] Results:
[INFO]
[INFO] Tests run: 140, Failures: 0, Errors: 0, Skipped: 0
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  32.646 s
[INFO] Finished at: 2020-02-12T20:44:10-08:00
[INFO] ------------------------------------------------------------------------

Commits

• My commits all reference appropriate Apache Helix GitHub issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation in the following wiki page:

(Link the GitHub wiki you added)

Code Quality

• My diff has been formatted using helix-style.xml

[huizhilu]

@narendly I pushed a new commit to fix unit tests, update tests to use namespaced APIs and remove deprecated classes. Tests all pass. Can you take a look again? Thanks.

[narendly]

Please address the comments :) Great work!

[huizhilu]

This PR is ready to be merged, approved by @narendly

Commit message:

We have metadata store directory service to help scale out zookeeper. Metadata store directory service provides REST APIs to access. This commit adds MSDS write endpoints to Helix REST.

Changelist:
- Add REST write endpoints to MetadataStoreDirectoryAccessor
- Add unit tests for the new REST write endpoints 
- Fix unit tests by cleaning up routing data path in ZK