Return an explicit and useful object in apply_async()

[diox]

The return value of PostRequestTask.apply_async(), when task queuing is enabled, is None instead of an AsyncResult. It makes sense since the task has not been sent yet, but it's annoying when you want to do something with the AsyncResult...

So ideally, we should return some kind of object instead, like a PostRequestTaskAsyncResult, which would be a proxy for AsyncResult. In that proxy object, accessing any property should cause us to bypass the task queuing, triggering the original apply_async() method, sending the task to the broker instead of waiting for the request to finish. We could then grab the AsyncResult and return the value of the property being accessed.