[WIP] Allow cancellation between reschedule() called and task resumed

[arthur-tacca]

Fixes #2973

• reschedule() gets a new parameter allow_abort: bool which, if True, still allows the abort function to be called right up until the task actually resumed. It is False by default so it's backwards compatible.
• When Event.wait() is woken by Event.set() it uses this parameter, so cancellation of it always succeeds

Not ready to merge I think the code is fine but:

• No tests
• No docs

[arthur-tacca]

As njsmith's comment says, the wait_task_reschedule() API is already complicated enough and this increases that for little demonstrable benefit. Closing.