[guava] If ListenableFuture is cancelled, it should cancel entangled coroutine before executing the listeners

[vadimsemenov]

This is related to ListenableFutures returned from Deferred.asListenableFuture and CoroutineScope.future.

Currently we override AbstractFuture.cancel and call super.cancel() before cancelling the coroutine.
This means that if the future has a slow listener, it can delay cancellation of the coroutine.

AbsractFuture explicitly recommends afterDone for this use case. We can use it when the API is finalised (at this moment afterDone is annotated with @Beta).

However, one disadvantage of afterDone, is that we'll have to unconditionally check if the future isCancelled(), which will cost us a volatile read even in case when the future is completed successfully.

See #2222 for more info, it added two @Ignored tests related to this issue.