acquiring has to be cancellable

[michaelficarra]

We thought that this may be the case, but I was really confronted with the need when implementing the summing combinator (any) in #9. The only way we can "race" Governors is to try to acquire all of them and then race the promises. Unfortunately, once we acquire the first one, we can't cancel the other attempts. Instead, we just have to queue up a release once they resolve. This wouldn't be so bad if we were only considering simple counting Governors. But since we want Governors to also be able to implement time-based rate limiting, acquiring and immediately releasing a token is not equivalent to aborting an acquisition attempt (or just never attempting to acquire). So I don't see any way around depending on some kind of cancellation.

[michaelficarra]

@lucacasonato and I worked on this a bit today.

• We should introduce a new interface, AbortableGovernor, that inherits from Governor.
• AbortableGovernor should have an acquireAbortable that returns both an abort callback and a promise for the token.
• If the abort callback is called before the token promise is fulfilled, the token promise is rejected.
• If the abort callback is called after the token promise has rejected, then no-op.
• If the abort callback is called after the token promise has resolved, ignore the cancellation request (also no-op).
• When a wrapped iterator is returned, it forwards the return and call the abort callbacks on pending token acquisitions from the governor.