Cancel loads of unused segments

[kfaraz]

As soon as a segment has been marked as unused, we need not continue loading it.

Changes

• Cancel loads of unused segments in UnloadUnusedSegments
• Add tests

Release note

Stop loading and moving segments as soon as they are marked as unused. This prevents historicals from wasting time on superfluous loads of segments that will be later unloaded anyway. Segments can be marked as unused either by a drop rule, by becoming overshadowed or through an API explicitly called by the user.

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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.