overlapping endpoint fixes in level compaction picker

[ajkr]

This diff addresses two problems. Both problems cause us to miss scheduling desirable compactions. One side effect is compaction picking can spam logs, as there's no delay after failed attempts to pick compactions.

1. If a compaction pulled in a locked input-level file due to user-key overlap, we would not consider picking another file from the same input level.
2. If a compaction pulled in a locked output-level file due to user-key overlap, we would not consider picking any other compaction on any level.

The code changes are dependent, which is why I solved both problems in a single diff.

• Moved input-level ExpandInputsToCleanCut into the loop inside PickFileToCompact. This gives two benefits: (1) if it fails, we will try the next-largest file on the same input level; (2) we get the fully-expanded input-level key-range with which we can check for pending compactions in output level.
• Added another call to ExpandInputsToCleanCut inside PickFileToCompact's to check for compaction conflicts in output level.
• Deleted call to IsRangeInCompaction in PickFileToCompact, as ExpandInputsToCleanCut also correctly handles the case where original output-level files (i.e., ones not pulled in due to user-key overlap) are pending compaction.

Test Plan: unit tests for the two problems mentioned

[facebook-github-bot]

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[sagar0]

seems reasonable to me.

[ajkr]

thanks a lot for the fast review! :)