Implement DataChangeListener

- Add a DataChangeReply to confirm receipt of a DataChange
- Created an AbstractUntypedActor class from which all actors extend. Currently the only thing this base
  class does is uniformly log incoming messages. Going forward will automatically register actors for DeathWatch and such.

Change-Id: Ibcc4179597023aa37b95641c0b666b3c650dc370
Signed-off-by: Moiz Raja <moraja@cisco.com>
Merge "Implement DataChangeListener"