Update Interfaces to match spec

[evankanderson]

Expected Behavior

The interfaces (particularly Subscribable) should match the spec from #482, see https://github.com/knative/eventing/blob/master/docs/spec/interfaces.md#subscribable in particular.

Actual Behavior

The implementation (which is currently over in pkg/) defines Subscribable and Channelable separately, and Sinkable and Targetable are not interchangeable.

Additional Info

This will probably also need to update the Subscription Controller and the in-memory Channel controller.

[adamharwayne]

/assign @adamharwayne

[adamharwayne]

I read through the spec again and found the following issues:

1. Old field names used in interfaces.md.
   • Update spec to match the new names from #563. #576 has merged.
2. dnsName seems like the wrong name. The examples are URIs and it is treated as a URI in the code. See this comment for more info.
   • My personal opinion is we should change the name of the field back to something that ends in URI (e.g. the original targetURI).
   • The other option is to change the examples and code to treat it as a DNS name.

[adamharwayne]

Another issue is that many of the events specified in the spec don't get emitted.

[adamharwayne]

Another issue is that the Subscribable interface is still being called Channelable. #565 fixes the name.
#565 has merged.