Refactor DOMDataBrokerImpl

This patch shuffles the abstractions, eliminating two intermediate
interfaces and ensuring proper layring is maintained with
AbstractDataBroker serving as a base for the Concurrent and Serialized
versions.

Change-Id: If363aa796db0c2ebf22fdefe7659dd428745dc75
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Refactor DOMDataBrokerImpl"