Summary:
This opens space for the new implementations of TransactionDBImpl such as WritePreparedTxnDBImpl that has a different policy of how to write to DB.
Closes https://github.com/facebook/rocksdb/pull/2689

Differential Revision: D5568918

Pulled By: maysamyabandeh

fbshipit-source-id: f7eac866e175daf3793ae79da108f65cc7dc7b25