Consumer won't overflow reporters now.

And also AsyncResults will be processed asap, instead of waiting for report call.
Now that you make me thing about it, ```7``` is kind of magical, without being sufficiently magical for my taste. :wink: 

I think you could just use ```None```, without even having an ```_END_WRITING``` constant? Just ```self._jobs.put(None)``` and ```if result is None:```...