Summary:
The new case is directIO = true, write_global_seqno = false in which we no longer write global_seqno to the external SST file.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4614

Differential Revision: D12885001

Pulled By: riversand963

fbshipit-source-id: 7541bdc608b3a0c93d3c3c435da1b162b36673d4