Add ability for historicals to clone an existing historical

[adarshsanjeev]

Description

Add the ability for Historicals to "clone" another existing Historicals, copying the segments they serve.
This would be useful for a case like rolling updates, where the goal is to launch a new historical as replacement for a currently existing one.

Sample config:

  "cloneServers": {"historicalA":"historicalB"}

The above config causes historicalA to be considered as an "unmanaged" node by the coordinator. This means that the node:

• Is not normally considered for segment assignment.
• Does not have its segments balanced.
• Is not counted towards replica counts as configured by load rules.

Coordinator manages historicalB as normal. However, any segment assignment or drop to historicalB is mirrored to historicalA.

If the mapping is removed, the coordinator once again manages historicalA during its duty cycles.

Release notes

• Added a new dynamic coordinator configuration, cloneServers, containing a map from target Historical server to source Historical server which should be cloned by the target. The target Historical does not participate in regular segment assignment or balancing. Instead, the Coordinator mirrors any segment assignment made to the source Historical onto the target Historical, so that the target becomes an exact copy of the source. Segments on the target Historical do not count towards replica counts either. If the source disappears, the target remains in the last known state of the source server until removed from the cloneServers.

Testing

• Added Coordinator simulation tests.
• Testing the changes on a local cluster with multiple historicals, and verified that segment changes to one are synced to the other.

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This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.

[kfaraz]

Overall, looks good to me. Left some initial feedback.

[kfaraz]

Thanks for the sim tests, @adarshsanjeev !
I have left some final comments.

[kfaraz]

@adarshsanjeev , please update the title and the description of the PR too.

[kfaraz]

Thanks for the detailed description, @adarshsanjeev !

[kfaraz]

@adarshsanjeev , found a couple of things that I had originally missed. Please let me know what you think.

[kfaraz]

LGTM 🚀 🚀