bpo-32710: Fix _overlapped.Overlapped memory leaks (GH-11489)

Fix memory leaks in asyncio ProactorEventLoop on overlapped operation
failures.

Changes:

* Implement the tp_traverse slot in the _overlapped.Overlapped type
  to help to break reference cycles and identify referrers in the
  garbage collector.
* Always clear overlapped on failure: not only set type to
  TYPE_NOT_STARTED, but release also resources.