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.

Lib/asyncio/selector_events.py

@@ -120,7 +120,10 @@ class BaseSelectorEventLoop(base_events.BaseEventLoop):
             try:
                 csock.send(b'\0')
             except OSError:
-                pass
+                if self._debug:
+                    logger.debug("Fail to write a null byte into the "
+                                 "self-pipe socket",
+                                 exc_info=True)
 
     def _start_serving(self, protocol_factory, sock,
                        sslcontext=None, server=None):