Provide a way to opt-out of stream and shutdown sematics

[TylerBloom]

Currently, all streams that get attached to the scheduler must be FusedStreams. This is because the scheduler will stop listening to a stream once it has yielded a None. It would be nice to provide support for some kind of fallible stream to allow users more direct access over when a stream should be considered done. This has some crossover with #2 .