Add ZooKeeperAccessor to helix-rest

[narendly]

Issues

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

Fixes #667
Fixes #668
Fixes #669

Description

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

In order to provide basic ZkClient functionalities (CRUD) in helix-rest, we want to add a ZooKeeperAccessor class that supports endpoints like get(), getChildren(), and exists().

Changelist:

1. Fix the double-checked locking idiom for byte array ZkBaseDataAccessor
2. Add ZooKeeperAccessor
3. Add tests

Tests

• The following tests are written for this issue:

TestZooKeeperAccessor.java

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

[INFO] Tests run: 93, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 21.96 s - in TestSuite
[INFO]
[INFO] Results:
[INFO]
[INFO] Tests run: 93, Failures: 0, Errors: 0, Skipped: 0
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 27.300 s
[INFO] Finished at: 2020-01-13T10:02:47-08:00
[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"

Code Quality

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

[jiajunwang]

LGTM!
Just one minor comment, which is optional.

[narendly]

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

[narendly]

Final commit message:

Add ZooKeeperAccessor to helix-rest

In order to provide basic ZkClient functionalities (CRUD) in helix-rest, we want to add a ZooKeeperAccessor class that supports endpoints like get(), getChildren(), and exists().

Changelist:

Fix the double-checked locking idiom for byte array ZkBaseDataAccessor
Add ZooKeeperAccessor
Add tests