BUG-1933: consult state in subtree modified path

resolveSubtreeChangeEvent() was missing a state.needsProcessing() check,
which means change resolution continued down that path eventhough no
listeners were subscribed to it. Add the check and return with a true if
we terminate early.

Change-Id: I7d386f3abd67c1a241ad88b5809236598dc967dd
Signed-off-by: Robert Varga <rovarga@cisco.com>