Cleanup segment metadata cache for a datasource if not in use

[kfaraz]

Follow up to #17824

This patch adds the implementation suggested in this comment

Changes

• Simplify the contract of DatasourceSegmentCache and SegmentMetadataCache
• Hold a reference on a HeapMemoryDatasourceSegmentCache while it is being used by a transaction
• Clean up unreferenced instances of HeapMemoryDatasourceSegmentCache in the sync thread
• Fix up tests

Read-Write Transaction flow

• Start JDBI transaction
• Create instance of SqlSegmentMetadataTransaction
• Within SegmentMetadataCache.writeCacheForDatasource():
  • Get or create a cache instance for the datasource
  • Acquire a reference to the cache instance
  • Acquire write lock on the cache instance
  • Perform the write operation
  • Invoke SqlSegmentMetadataTransaction.close()
  • Release write lock on the cache instance
  • Release the reference to the cache instance
• Finish JDBI transaction

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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]

Thanks a lot for the review, @gianm !!