Summary:
Blob db rely on base db returning sequence number through write batch after DB::Write(). However after recent changes to the write path, DB::Writ()e no longer return sequence number in some cases. Fixing it by have WriteBatchInternal::InsertInto() always encode sequence number into write batch.

Stacking on #2375.
Closes https://github.com/facebook/rocksdb/pull/2385

Differential Revision: D5148358

Pulled By: yiwu-arbug

fbshipit-source-id: 8bda0aa07b9334ed03ed381548b39d167dc20c33