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: Ia27f2570314b3016647efb89a3f2c2d35d68a92a
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: Ibe8427f516bea9135b34dc9253edda0e79cddd7e
Signed-off-by: Moiz Raja <moraja@cisco.com>