2.x Design: Creation/Destruction

How to create and destroy streams.

The wording of this can become much better once agreement occurs in https://github.com/ReactiveX/RxJava/pull/3348/files

The intent of this is:

1) Specify how to create streams (Observable/Flowable) of the various flavors, and account for flow control.
2) Evolve towards what the creation/generation APIs are
3) Specify that it is not the responsibility of the final Subscriber to call unsubscribe/cancel upstream