Fix ConcurrentModification exception in Workflow Garbage Collection

[alirezazamani]

Issues

• My PR addresses the following Helix issues and references them in the PR title:
  Fixes ConcurrentModificationException for the TaskGarbageCollectionStage #738

Description

• Here are some details about my PR, including screenshots of any UI changes:
  In workflow Garbage collection, there is possibility that we see ConcurrentMod exception
  while looping through the contexts. The reason behind this exception is that the contexts can be modified using other thread (i.e. controller's main thread or other Garbage Collection threads). This commit fixes this issue by first making a copy of the contexts and then leveraging try catch to avoid such exceptions.

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

[ERROR] Tests run: 1083, Failures: 1, Errors: 0, Skipped: 2, Time elapsed: 4,194.441 s <<< FAILURE! - in TestSuite
[ERROR] testSubscribeDataChange(org.apache.helix.manager.zk.TestZKWatch) Time elapsed: 0.023 s <<< FAILURE!
java.lang.AssertionError: expected:<0> but was:<1>
at org.apache.helix.manager.zk.TestZKWatch.testSubscribeDataChange(TestZKWatch.java:81)

[INFO]
[INFO] Results:
[INFO]
[ERROR] Failures:
[ERROR] TestZKWatch.testSubscribeDataChange:81 expected:<0> but was:<1>
[INFO]
[ERROR] Tests run: 1083, Failures: 1, Errors: 0, Skipped: 2
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 01:09 h
[INFO] Finished at: 2020-02-10T13:27:31-08:00
[INFO] ------------------------------------------------------------------------

mvn test -Dtest="TestZKWatch" result:
[INFO] Tests run: 3, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.918 s - in org.apache.helix.manager.zk.TestZKWatch
[INFO]
[INFO] Results:
[INFO]
[INFO] Tests run: 3, Failures: 0, Errors: 0, Skipped: 0
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 6.069 s
[INFO] Finished at: 2020-02-10T13:27:54-08:00
[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

[narendly]

Thank you!

[alirezazamani]

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

Final commit message:
Title:
Fix ConcurrentModification exception in Workflow Garbage Collection

Body:
In workflow Garbage collection, there is possibility that we encounter
ConcurrentMod exception while looping through the workflow contexts.
This commit fixes this issue by looping through a deep copy of the
contexts list.