Add customized threadpool

[mgao0]

Issues

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

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Description

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Tests

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Changes that Break Backward Compatibility (Optional)

• My PR contains changes that break backward compatibility or previous assumptions for certain methods or API. They include:

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Documentation (Optional)

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Commits

• My commits all reference appropriate Apache Helix GitHub issues in their subject lines. In addition, my commits follow the guidelines from "How to write a good git commit message":
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  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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