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``?
I think that the updated `enqueue_request` API (returning False if request was not scheduled and True if it was) makes sense and it is better than just returning None.