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>
Merge "Implement TransactionProxy#read"