BUG-997: migrate the global context schema service

This change test-drives the new re-implementation, making sure we have a
user. An immediate advantage should be faster context convergence times.
Requires I3311a3f0251868f91c385d0291365934ccfb9ede in yangtools.

Change-Id: Ic2f07144043f456ab9821013c1d690904c9a791f
Signed-off-by: Robert Varga <rovarga@cisco.com>