Bug 500: DOMStore SPI

Bug 500: DOMStore SPI

This is the API which needs to be implemented by a single logical tree
store, such that it can be plugged into the in-memory datastore
prototype.

The prototype keeps one logical data store for each of operational and
configuration subtrees. A front-end (client-visible) transaction results
in two back-end (data store) transactions. State transitions between the
three are coordinated using three-phase-commit protocol (3PC), with the
frontend transaction acting as the coordinator and the backend
transactions acting as cohorts.

Change-Id: Idfce04553e7c36ae6a1bb8c8b2699ca78c458bb4
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>
Signed-off-by: Robert Varga <rovarga@cisco.com>