Summary:
Periodic compactions ensure that even SSTs that do not get picked up
otherwise eventually go through compaction; used in conjunction with
BlobDB's garbage collection, they enable BlobDB to reclaim space when
old blob files are used by such straggling SSTs.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6172

Test Plan: Ran `make check` and used the BlobDB mode of `db_bench`.

Differential Revision: D19045045

Pulled By: ltamasi

fbshipit-source-id: 04636ecc4b6cfe8d495bf656faa65d54a5eb1a93