DelayedStream.setStream() should cancel the provided stream if not using it

If `cancel()` or `setStream()` was previously called then `setStream()` throws away the stream argument. It should `cancel()` the stream to make sure resources are freed, although I think only DelayedClientTransport benefits.

This should not be done until after #1536. I don't know if OkHttp suffers a similar problem. If so, we may want to revisit whether it makes sense to allow streams to be cancelled before `start()`.