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 #3050 : _controllerOpt getting shared across resources

Description

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  Refer to _controllerOpt getting shared across resources #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

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  Added a new test testNoSharedLockAcrossInstances
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Code Quality

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[LZD-PratyushBhatt]

@junkaixue @GrantPSpencer Can you take a look at this? Thanks!

[GrantPSpencer]

LGTM. Great catch on this, will greatly reduce topstate handoff when controller is processing multiple STs simultaneously

Just have 1 nit comment on test assertion

[GrantPSpencer]

LGTM
will still need committer approval, cc @junkaixue @xyuanlu

[LZD-PratyushBhatt]

Hey @junkaixue can you take a look at this again if you get time, Thanks a lot!

[LZD-PratyushBhatt]

Hey @junkaixue , @xyuanlu reminder on this one, Thanks for your time!