Implement rest of the methods for TransactionProxy

Implement the following methods,
  - write
  - merge
  - delete
  - getIdentifier
  - ready
  - close

Introduce the ThreePhaseCommitCohortProxy

Change-Id: I8d6d58a9965966d3fa6f1230761480b15a0abc3c
Signed-off-by: Moiz Raja <moraja@cisco.com>
Merge "Implement rest of the methods for TransactionProxy"