Immediate unsubscription of signals

Currently, we queue the match rule unsubscription in the sync Drop impl of MessageStream as we don't have an async equivalent. This means the users have no way of either unsubscribing from signals immediately or even an easy/simple way to be notified of the subscription. It should be relatively easy to provide an async API that unsubscribes from the match rules immediately. This should be added for both MessageStream and SignalStream.