Automatically wait for promises on destruct.

[Nyholm]

From https://github.com/php-http/httplug-quickstart/pull/1#issuecomment-312874509

Currently Guzzle automatically calls the queue in the shutdown handler or can be called manually. The queue is responsible for waiting on Promises. In our cURL client the only solution right now is to manually do that for every promise.

[sagikazarmark]

Not sure if this is precise. On destruct of what? The client? That would require the keep the queue inside the client which I am not sure we want.