Adds the request for execution. A strong reference of the request is kept internally while the request is running. It is an error to drop the request object without calling Request->Cancel() if there is a notification delegate bound to the request. The delegate will be called from a worker thread even if you have released the request because of the strong reference being held internally. If your delegate callback object has been destroyed it may result in a crash. You must hold on to the request until the callback signals completion or you have to call Cancel().