refactor(Subscriber): Observer should just be an interface

[benlesh]

I think we could change Subscriber to be a class that extends Subscription and implements Observer rather than the other way around. Then Observer could just be an interface, and we can clean up some logic most likely.

The reason I suggest this is we never actually new up an Observer anywhere... whereas Subscription does get new-ed up and reused frequently. So it doesn't make as much sense to have Subscription be the one that is being used like an interface on Subscriber.

cc/ @trxcllnt