Gateway util for per key lock and per key blocking queue executor

[xyuanlu]

Issues

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

Gateway util for per key lock and per key blocking queue executor

Description

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Add per key lock and per key blocking queue executor

Tests

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• 2 new test class
  TestPerKeyBlockingExecutor and TestPerKeyBlockingExecutor

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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")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Code Quality

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[xyuanlu]

This change is ready to be merged. Approved by @zpinto
Commit message: [Gateway util for per key lock and per key blocking queue executor]