Summary:
This change models Optimistic Tx db after Pessimistic TX db. The motivation for this change is to make the ptr polymorphic so it can be held by the same raw or smart ptr.

Currently, due to the inheritance of the Opt Tx db not being rooted in the manner of Pess Tx from a single DB root it is more difficult to write clean code and have clear ownership of the database in cases when options dictate instantiate of plan DB, Pess Tx DB or Opt tx db.
Closes https://github.com/facebook/rocksdb/pull/3566

Differential Revision: D7184502

Pulled By: yiwu-arbug

fbshipit-source-id: 31d06efafd79497bb0c230e971857dba3bd962c3