Fix org.opendaylight.yangtools.concepts.Registration

The concept of Registration is currently tied to a particular object – as it is recognized by its abstract implenentation which is called AbstractObjectRegistration.

We need to remove the getInstance() method to make the concept reusable in all situations. Unfortunately this requires synchronizing with users of our APIs.

Cautious plan of record:
1) 0.6.2: introduce ObjectRegistration, which simply inherits from Registration
2) 0.6.3: remove Registration completely, such that only ObjectRegistration exists
3) 0.6.4: reintroduce Registration without the getInstance() method

But we may want to accelerate the process, as the generic Registration would be very useful in the BGPCEP project.