Add cancelling capability to push non-backpressure aware path

[johnmcclean]

Currently non-backpressure aware Streams run until the event stream itself stops

[johnmcclean]

This can be implemented simply & performantly in the same way the short circuiting is implemented in allMatch etc.

[johnmcclean]

Create a specific Throwable with FillInStackTrace overwriten to represent a cancelled Stream, handle the cancellation in each Operator (throwing it upwards) and finally mark the Stream as cancelled at the top level in Spouts.

[johnmcclean]

Doesn't seem to feasible as many of the key APIs are user controlled for convenience (e.g. Spouts.async)