Disconnect NetconfNotificationCollector registration

Disconnect NetconfNotificationCollector registration

NotificationRegistration does is actually the same thing as
Registration, except it does not enforce idempotence. Replace its use in
NetconfNotificationCollector with Registration and use an explicit
AbstractObjectRegistration subclass.

This fixes potential problems with multiple (un)registrations of the
same object and opens up the possibility for future removal of locking,
as AbstractObjectRegistration knows whether it is closed or not.

Change-Id: I1ed8e1f74ee755d344815c511499a8dbfc9cce8d
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>