An attempt to resolve #977, add signal to be sent when request is dropped by the scheduler  

There are some ideas about how to solve the issue #977 in this patch. I am new to the project and thus may have missed something here.

Is it better to name the signal in a more specific way, eg ``request_dropped_by_dupefilter``?
What about adding a method to the scheduler which will validate the requests? Something like ``is_request_valid``. Also, it can be called twice (in engine and in ``enqueue_request`` itself) to preserve backwards compatibility). I do not see troubles in current approach though - as far as I can see the result of this function is not used anywhere.