Skip one time listener re-register for exists

[xyuanlu]

Issues

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

#2237

Description

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This PR skips one time listener re-register for exists()

Tests

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testChangeListener

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Changes that Break Backward Compatibility (Optional)

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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":
  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

• My diff has been formatted using helix-style.xml
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[xyuanlu]

This change is ready to be merged. Approved by @Marcosrico
Commit message: Skip one time listener re-register for exists for ZkClient - MetaClient usage.