BUG-2673: make CDS implement DOMDataTreeChangeListener

This patch adds the base support for registering
DOMDataTreeChangeListeners. These are delivered only on the local node,
as efficient serialization requires interaction with the cluster
topology and shard leadership handoff. That functionality will be
delivered in a follow-up patch.

It also introduces a bit common infrastructure to be used by
DataChangeListener code, as it performs a very similar function.

Change-Id: Ifb91d08857684fb160fd923bc25c294d2fca4bc3
Signed-off-by: Robert Varga <rovarga@cisco.com>