Summary:
This patch refactors TransactionImpl by separating the logic for pessimistic concurrency control from the implementation of how to write the data to rocksdb. The existing implementation is named WriteCommittedTxnImpl as it writes committed data to the db. A template named WritePreparedTxnImpl is also added which will be later completed to provide a an alternative implementation.
Closes https://github.com/facebook/rocksdb/pull/2676

Differential Revision: D5549998

Pulled By: maysamyabandeh

fbshipit-source-id: 16298e86b43ca4849324c1f35c731913c6d17bec