'Clear any expired state from memory.'

def _wipe_expired(self):

    current_time = datetime.utcfromtimestamp(self._clock.seconds())
    evolver = self._information_wipers.evolver()
    for (key, wipe) in self._information_wipers.items():
        last_activity = wipe.last_activity()
        if ((current_time - last_activity) >= EXPIRATION_TIME):
            self._deployment_state = wipe.update_cluster_state(self._deployment_state)
            evolver.remove(key)
    self._information_wipers = evolver.persistent()
