(Merge 3.4) asyncio: sync with Tulip

* Tulip issue #196: IocpProactor._poll() clears the reference to the
  overlapped operation when the operation is done. It would be better to clear
  the reference in a new _OverlappedFuture.set_result() method, but it cannot
  be done yet because of a weird bug.
* BaseSelectorEventLoop._write_to_self() now logs errors in debug mode.