State assignment for SWAP_IN replicas to only assign top state if state model has "R" or "N"

[zpinto]

Issues

Make logic to determine state of replicas on SWAP_IN instance simpler and more predictable during an in-flight node swap. #2662

Description

Changing state assignment logic for replicas on swap in node to only be assigned top state if the state model says that all replicas or all candidate nodes can have the top state. All other cases will have the second top state assigned. Also, moving N and R to constants and updating code where necessary.

Tests

• Current tests cover this change.

Changes that Break Backward Compatibility (Optional)

NA

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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  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)

[zpinto]

Test failed: testCacheDataUpdates(org.apache.helix.metaclient.impl.zk.TestZkMetaClientCache) #2693 which is now fixed by #2705

This PR is ready to be merged.

Final Commit Message:
Make logic to determine state of replicas on SWAP_IN instance simpler and more predictable during an in-flight node swap.