Move existing assignments usage calculation to pre-process stage

[frankmu]

Issues

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

resolves #2822

Description

• Here are some details about my PR, including screenshots of any UI changes:

One of the key steps of sticky rebalance strategy is to populate the previous node usage based on current mapping. Previous we do this in rebalance stage where we don't have the node usage info from global view. e.g. if the nodeA has resource1, then in rebalance stage of resource2, we don't have this info anymore. This PR moves the existing assignments usage calculation to pre-process stage where we will have the global view for all resources.

Will add one more test in following PR when #2889 is merged from master to this feature branch.

Tests

• The following tests are written for this issue:

helix-core/src/test/java/org/apache/helix/integration/rebalancer/TestStickyRebalanceStrategy.java

• The following is the result of the "mvn test" command on the appropriate module:

mvn test -o -Dtest=TestStickyRebalanceStrategy -pl=helix-core
...
[INFO] Tests run: 9, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 61.9 s - in TestSuite
[INFO] 
[INFO] Results:
[INFO] 
[INFO] Tests run: 9, Failures: 0, Errors: 0, Skipped: 0
[INFO] 
[INFO] 
[INFO] --- jacoco:0.8.6:report (generate-code-coverage-report) @ helix-core ---
[INFO] Loading execution data file /Users/tmu/projects/helix/helix-core/target/jacoco.exec
[INFO] Analyzed bundle 'Apache Helix :: Core' with 961 classes
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
...
mvn test -o -Dtest=TestStickyRebalanceWithGlobalPerInstancePartitionLimit -pl=helix-core
...
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 19.285 s - in org.apache.helix.integration.rebalancer.TestStickyRebalanceWithGlobalPerInstancePartitionLimit
[INFO] 
[INFO] Results:
[INFO] 
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
[INFO] 
[INFO] 
[INFO] --- jacoco:0.8.6:report (generate-code-coverage-report) @ helix-core ---
[INFO] Loading execution data file /Users/tmu/projects/helix/helix-core/target/jacoco.exec
[INFO] Analyzed bundle 'Apache Helix :: Core' with 961 classes
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS

Changes that Break Backward Compatibility (Optional)

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

(Consider including all behavior changes for public methods or API. Also include these changes in merge description so that other developers are aware of these changes. This allows them to make relevant code changes in feature branches accounting for the new method/API behavior.)

Documentation (Optional)

• In case of new functionality, my PR adds documentation in the following wiki page:

(Link the GitHub wiki you added)

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
  (helix-style-intellij.xml if IntelliJ IDE is used)

[junkaixue]

good job!

[frankmu]

This PR is ready to be merged. Approved by @junkaixue
Commit message: [Move existing assignments usage calculation to pre-process stage]