CDS: use internal DataTree instance

CDS: use internal DataTree instance

Instead of using a captive InMemoryDataStore instance, which is geared
toward interaction with a DOMDataBroker, use the baseline DataTree
instance and utility classes for implementing the data store behavior.

Since most of CDS is geared for interactions with the usual DOMStore
interfaces, this patch provides a set of bare-bone workalike classes,
but taking advantage of the safeties provided by the AKKA actor system.

Immediate benefit should be a speed up in backend processing, as we
eliminate intermediate Future instances when accessing and manipulating
data.

We also gain explicit control over DataTree lifecycle, which enables us
switching to DataTreeCandidate as the replication and persistence unit,
as opposed to individual modifications.

Change-Id: I031d0ed4d82dcc923fd377c13fefba2819923f9b
Signed-off-by: Robert Varga <rovarga@cisco.com>