Refactor ProcessKeyValueCompaction into smaller functions (#13879)

Summary:
**Context/Summary:**
`ProcessKeyValueCompaction()` has grown too long to resonate or add any logic to resume from some key and save progress for resumable compaction. This PR breaks this function into smaller functions. Almost all of them are cosmetic changes, except for one thing pointed out in below PR conversation.

Specially, this PR did the following:
- Added `SubcompactionInternalIterators`, `SubcompactionKeyBoundaries` and `BlobFileResources` to manage the lifetime of the local variables of the original functions to be used across smaller functions
- Moved AutoThreadOperationStageUpdater, some IO stats measurement to a different place that makes more sense

Pull Request resolved: #13879

Test Plan: Existing UT

Reviewed By: jaykorean

Differential Revision: D80216092

Pulled By: hx235

fbshipit-source-id: 515615906e5e5fd5ec191bcdd4126f17d282cac2

Author: hx235