[Snapshot] Improve KeyDeletingService to reclaim eligible key blocks in snapshot's deletedTable

Deep cleaning snapshot's deletedTable and deletedDirectoryTable.

From Snapshot GC design doc §4.d)2.

If the active filesystem’s deletedTable is already empty, the background task can optionally start walking back the snapshot chain for each bucket in order to start cleaning up the deletedTable in the existing DB checkpoints. Iterate over each DB checkpoint’s deletedTable ..