acquire segment references in bulk for ServerManager to simplify reference handling

[clintropolis]

Description

Follow-up to #18005, this PR changes ServerManager to acquire segment references in bulk before building query runners instead of lazily during the processing of the segment. I think that this should also help reduce the chance for segments to be dropped before being processed since grabbing the reference for each segment is not dependent on when that segments runner runs, in exchange for holding the reference a bit longer. ReferenceCountingSegmentQueryRunner is no longer needed and has been deleted.

Also did some minor code cleanup in SegmentManager to tidy things up (no logic changes).

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This PR has:

• been self-reviewed.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• 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.