Summary:
During bottommost compaction, RocksDB cannot simply drop a tombstone if
this tombstone is not in the earliest snapshot. The current behavior is: RocksDB
skips other internal keys (of the same user key) in the same snapshot range. In
the meantime, RocksDB should check for the `shutting_down` flag. Otherwise, it
is possible for a bottommost compaction that has already started running to take
a long time to finish, even if the application has tried to cancel all background jobs.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7356

Test Plan: make check

Reviewed By: ltamasi

Differential Revision: D23663241

Pulled By: riversand963

fbshipit-source-id: 25f8e9b51bc3bfa3353cdf87557800f9d90ee0b5