Support preserving history of HELIX_ENABLED legacy fields when combo of instance operation and old helix version are used

[zpinto]

Issues

• Support preserving history of HELIX_ENABLED legacy fields when combo of instance operation and old helix version are used

Description

When the new instance operation APIs are used, we need to consider that an older version of helix could have been used to modify the legacy fields of HELIX_ENABLED, HELIX_DISABLED_TYPE, and HELIX_DISABLED_REASON. When this happens, we need to write those legacy fields to the USER source instance operation to preserve the history when another source sets the instance operation. When the other source re-enables the instance, we should restore the old legacy fields with what was previously set.

Tests

• Added to test TestInstanceConfig#testEnabledInstanceBackwardCompatibility

Changes that Break Backward Compatibility (Optional)

• Yes, will add more information

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)

[xyuanlu]

Thanks for the change.
I would suggest separate the formatting and logic change for easier review. :D

[junkaixue]

The pr is too large for reviewing combining format change + logic change. Please split the PR with purely logic change + refactoring.

[junkaixue]

Also, this is related to instance operations. Highly recommend to fix the unstable instance operation test as well. It is vulnerable there for a long while.

[xyuanlu]

Overall looks good. Only one minor comment.

[zpinto]

This PR is ready to be merged.

Commit message:
When the new instance operation APIs are used, we need to consider that an older version of helix could have been used to modify the legacy fields of HELIX_ENABLED, HELIX_DISABLED_TYPE, and HELIX_DISABLED_REASON. When this happens, we will write those legacy fields to the USER source instance operation to preserve the history when another source sets the instance operation. When the other source re-enables the instance, we should restore the old legacy fields with what was previously set.