Split out BindingDataObjectCodecTreeNode.streamChild()
BindingCodecTree does not have a way of entering the tree without
specifying an InstanceIdentifier construct. This is a mistake, as we
need more powerful addressing capabilities to be able to deal with
Notifications and other constructs.
SchemaRootCodecContext is then taught to handle Notifications specially,
now that they cannot legally come from InstanceIdentifier.
JIRA: MDSAL-730
Change-Id: I970cab36d2794472ef801cd0e0d67c264bd169f4
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>