Initial implementation of the Shard Actor

Initial implementation of the Shard Actor

Things to note,
- Added a temporary dependency on sal-broker-impl. This is so that I could use the InMemoryDOMDataStore.
  Once InMemoryDOMDataStore is moved to it's own bundle then I will simply switch the dependency
- Shard has been only implemented to the point where it is using an InMemoryDOMDataStore and handling the CreateTransaction and RegisterChangeListener messages
  This commit is intended to give a feel for what kind of coding patterns will be used to implement Shard and related actors and their tests

Change-Id: I86f0d701399805185a0987bb1b97fe1358ce4cd9
Signed-off-by: Moiz Raja <moraja@cisco.com>