Convert ProxyDOMDataBroker tx creation to async

Convert ProxyDOMDataBroker tx creation to async

ProxyReadWriteTransaction was modified to take the ActorRef
Future from the tx creation message and add a callback when
complete. Transaction operations prior to completion are
queued and replayed to a ProxyTransactionFacade instance
once the Future is complete. For successful completion, an
ActorProxyTransactionFacade is created that interfaces
with the master ActorRef. On failed Future, a
FailedProxyTransactionFacade is created that returns a
failed Future for read, exists and commit operations.

The ProxyReadAdapter and ProxyWriteAdapter were removed
in lieu of ProxyTransactionFacade which combines the
read/write operations for simplicity.

Some simple actor response messages,
eg NewWriteTransactionReply, were removed in lieu of
returning the payload or exception via
akka.actor.Status.Success or akka.actor.Status.Failure.
This simplifies caller response handling as akka
automatically unwraps Success and Failure in callbacks
and synchronous await.

UTs were added, modified and removed corresponding to
the changes.

Change-Id: I0bd2a931d91ded97ebba7ccc207d51bd6474a41c
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>