BUG-1120: fix race window in unregistration

This has the nice benefit of unifying the registration/unregistration
paths, too. This requires a supporting ehnancement in yangtools, which
has been submitted at https://git.opendaylight.org/gerrit/#/c/7568/

Change-Id: Ifd489d6eaf5b7feb4a6efaa09bd468d5818c24cc
Signed-off-by: Robert Varga <rovarga@cisco.com>