Report non-present initial data in (DOM)DataTreeChangeListener

Report non-present initial data in (DOM)DataTreeChangeListener

Add a default method 'onInitialData' with no-op in
DOMDataTreeChangeListener', which would be invoked only once during
registration of the listener if there was no data in the conceptual
data tree for the supplied path, which was used to register this
listener, and after this 'onDataTreeChanged' would always be
invoked for data changes.

Users not caring about this event could leave it as default with no-op.

JIRA: MDSAL-217
Change-Id: I3a2c02c76b8c37ac7624dd1646692bd783a3468d
Signed-off-by: Jie Han <han.jie@zte.com.cn>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>