Fix global lock contention in DistClusterControllerStateModel caused by Optional.empty() singleton

[LZD-PratyushBhatt]

Issues

• My PR addresses the following Helix issues and references them in the PR description:
  This PR fixes _controllerOpt getting shared across resources apache/helix#3050 : _controllerOpt getting shared across resources

Description

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  Apache Helix PR: Fix global lock contention in DistClusterControllerStateModel caused by Optional.empty() singleton apache/helix#3052

Refer to apache#3050 for Problem description.
Basically, all DistClusterControllerStateModel instances were synchronizing on the same global lock object due to Optional.empty() returning a singleton instance.
Replaced synchronization on _controllerOpt (the shared singleton) with a dedicated per-instance lock object _controllerLock = new Object(). This eliminates cross-instance contention while preserving within-instance synchronization guarantees.
(Write a concise description including what, why, how)

Tests

• The following tests are written for this issue:
  Added a new test testNoSharedLockAcrossInstances
  (List the names of added unit/integration tests)

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

(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
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  4. Subject uses the imperative mood ("add", not "adding")
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Code Quality

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[proud-parselmouth]

Nit: Add the reference to the apache helix PR. Would be easier to reference any comments there.

[proud-parselmouth]

LGTM. thanks for this.

[LZD-PratyushBhatt]

Test failure is unrelated.

[info] ./helix-core/target/surefire-reports/TestSuite.txt: Tests run: 1466, Failures: 1, Errors: 0, Skipped: 3, Time elapsed: 6,694.226 s <<< FAILURE! - in TestSuite
Error:  Test failed: testDistributedController(org.apache.helix.integration.manager.TestConsecutiveZkSessionExpiry)  Time elapsed: 34.064 s  <<< FAILURE!

[proud-parselmouth]

You can link the apache helix flaky test issue with your comment.

apache#2882