Fix Empty IdealState Calculation with Disabled Nodes in AutoRebalance…

[MarkGaox]

…Strategy

Issues

• My PR addresses the following Helix issues and references them in the PR description:
• AutoRebalanceStrategy generate empty IdealState #2876

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Description

• Here are some details about my PR, including screenshots of any UI changes:
  (Write a concise description including what, why, how)

When calculating the preferred assignment, we should skip the node if the node is already disabled.

• made above in

private Map<Replica, Node> computeExistingPreferredPlacement(
     final Map<String, Map<String, String>> currentMapping) {

• refine the naming in AutoRebalanceStrategy to follow the naming conventions.

Tests

• The following tests are written for this issue:

mvn clean install -Dmaven.test.skip.exec=true && mvn test -o -Dtest=TestAutoRebalanceStrategy -pl=helix-core

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":
  1. Subject is separated from body by a blank line
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Code Quality

• My diff has been formatted using helix-style.xml
  (helix-style-intellij.xml if IntelliJ IDE is used)

[xyuanlu]

May I suggest separate the logic change and style change? It is easier to review and track change history.

[junkaixue]

Split the PR with fix + refactor.

[junkaixue]

let @zpinto take another look?

[junkaixue]

@MarkGaox can you follow up with Zac?

[zpinto]

LGTM

[MarkGaox]

This PR is ready to merge. Approved by @junkaixue @zpinto

Commit message: Fix IdealState Calculation in AutoRebalanceStrategy for Disabled Nodes