Disable QSocketNotifier during reader/writer callbacks + ignored finished Futures

In order to integrate properly with ZeroMQ, I discovered I had to disable QSocketNotifier before polling a corresponding ZeroMQ socket. I decided to do this before calling reader/writer callbacks registered with add_reader/add_writer, since this is typically where you poll ZeroMQ. I hope this won't cause problems for any other scenarios, but I can't see why it should, really.

It also turned out Python 3.4.2 unearthed some issues due to mutating Futures that are in the finished state, which I've taken care of.