Implement DistributedDataStore#registerDataChangeListener

Change-Id: I153c76b923dff7845321699d556f30f2ecadec57
Signed-off-by: Moiz Raja <moraja@cisco.com>
Implement TransactionProxy#read

- Move utility code into ActorContext
- Create a reusable MockActorContext for tests
- Update DistributedDataStore to use the common code
- Update TransactionChainProxy to construct the appropriate TransactionProxy's

Change-Id: I30386d2370f2683020e05d8a3842c0c63708df6f
Signed-off-by: Moiz Raja <moraja@cisco.com>
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 DistributedDataStore#registerDataChangeListener"
Merge "Implement TransactionProxy#read"
Merge "Implement rest of the methods for TransactionProxy"