Define smaller interfaces for methods in adjudcator

[manoranjith]

• Previously, the adjudicator interface consisted of four methods.

• But, only some of the methods were required in any particular place.

• Hence, define an interface for each of the methods and compose the
  adjudicator interfaces from these smaller interfaces.

Resolves #184.

[matthiasgeihs]

Some comments on the interface definitions. Let's discuss during our call later.

I have also some comments on the documentation, but we can do that afterwards.

[manoranjith]

Some comments on the interface definitions. Let's discuss during our call later.

I have also some comments on the documentation, but we can do that afterwards.

@matthiasgeihs As discussed in the call, I have revised the docs comments on the interfaces to idiomatic style (as used in the go standard library for for interfaces like io.Writer, io.WriteSeeker etc.,)

Also, I have removed the RegisterSubscriber method, which can be added as the part of the PR which uses that interface.

[matthiasgeihs]

OK, so far this looks fine. However, I would like to wait with approval and merging until we have a complete draft of the implementation of the local watcher. I am not super happy about breaking up the adjudicator interface into many single-method interfaces. It might make sense to only separate those which are relevant to the watcher.

[matthiasgeihs]

Closed in favor of #212