Event implementation is rewritten to match threading.Event more closely

- Event.set() does not accept a value anymore; Event.get() is gone
- "Event.set(); Event.clear()" now works as expected and does notify the
  waiters; it used to be a no-op with the old implementation
- AsyncResult does not inherit from Event anymore but it inherited all
  the code of the old Event. It does not have clear() method anymore.
  Otherwise it works exactly like before.