BUG-1704: expose proper SchemaNode subclasses

SchemaTracker users may actually need the correct subclasses. Since the
tracker already does the proper checks, this is a simple, compatible
change.

Change-Id: I24e2bb0f7a0598435879d5ed234bc4ab75da571a
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-1704: expose proper SchemaNode subclasses"