Add close method to Helix lock

[mgao0]

Issues

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

Currently there is no close method to close the lock and the underlying zk client connection. It could result to zk client leak.

Description

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

Added a close() method to the interface and implementation of the lock, where the zk client will be closed. User will need to call this when they don't need to use the lock any more to avoid zk connection leakage.

Tests

• The following tests are written for this issue:
  TestZKHelixNonblockingLock.testCloseLockedLock
  TestZKHelixNonblockingLock.testCloseUnlockedLock

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

[INFO] Results:
[INFO]
[INFO] Tests run: 6, Failures: 0, Errors: 0, Skipped: 0
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 53.817 s
[INFO] Finished at: 2020-06-09T16:09:54-07:00
[INFO] ------------------------------------------------------------------------

Commits

• My commits all reference appropriate Apache Helix GitHub issues in their subject lines. 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
  (helix-style-intellij.xml if IntelliJ IDE is used)

[mgao0]

This PR is ready to be merged, approved by @jiajunwang.
Final commit message: Add a close method to Helix lock interface and current implementation to avoid ZK connection leakage