Remove a potential deadlock when shutting down a RoutingTableProvider.

[jiajunwang]

Issues

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

Fixes #1748

Description

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

This PR aims to avoid a potential deadlock that happens in a race condition when shutting down RoutingTableProvider.

The race condition happens when the RoutingTableProvider is shut down and the corresponding HelixManager is processing an onLiveInstanceChange event. Both threads require 2 locks, HelixManager lock, and RoutingTableProvider._lastSeenSessions, in different orders.
To resolve this problem, this PR changes 2 logic.

1. Remove the _lastSeenSessions lock usage. So no more deadlock. The logic correctness is now ensured by the AtomicReference.getAndSet method.
2. Remove LiveInstanceChangeListener in the shutdown call before processing the other logic. This is a missing logic and should be done regardless of the deadlock problem. Moreover, this change also avoids a more complicated race condition introduced by the previous change that may lead to CurrentStates callback handler leakage.

Test cases in TestZkCallbackHandlerLeak have been updated to validate the logic changes.

Tests

• The following tests are written for this issue:

TestZkCallbackHandlerLeak

I run the test over the weekend, passed continuously 3533 times already.

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(If CI test fails due to known issue, please specify the issue and test PR locally. Then copy & paste the result of "mvn test" to here.)

Changes that Break Backward Compatibility (Optional)

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Documentation (Optional)

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Commits

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  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")
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  6. Body explains "what" and "why", not "how"

Code Quality

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[junkaixue]

Overall, looks good

[jiajunwang]

Approved by @junkaixue , I will merge the PR.