Use num cores to determine balancerComputeThreads

[kfaraz]

Changes

• Determine the default value of balancerComputeThreads based on number of coordinator cpus rather than number of segments. Even if the number of segments is low and we create more balancer threads, it doesn't hurt the system as threads would mostly be idle.
• Remove unused field from SegmentLoadQueueManager

Expected values

• Clusters with ~1M segments typically work with Coordinators having 16 cores or more. This would give us 8 balancer threads, which is the same as the current maximum.
• On small clusters, even a single thread is enough to do the required balancing work.

Release note

The coordinator dynamic config now uses num_cpus / 2 as the default value for balancerComputeThreads instead of the old value of 1.
This is also the value used when smartSegmentLoading is enabled.

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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.