Proposal: Add cancelAll() method to FIFOQueue/ActorQueue

[songhuaixu]

Goals

Provide a thread-safe method to cancel all pending operations in a queue

Non-Goals
No changes to Queue's fundamental execution model

Does not handle operation dependencies automatically

Investigations

Individual cancellation: Requires maintaining references to all Task and calling cancel() on each, which is cumbersome

[dfed]

Thanks for the idea! Curious to get your thoughts on #63

[songhuaixu]

It's not quite what I imagined, but it's still amazing.