Push entry onto our access log, invalidate the old entry if exists.

def _push_entry(self, key):
        "Push entry onto our access log, invalidate the old entry if exists."
        self._invalidate_entry(key)

        new_entry = AccessEntry(key)
        self.access_lookup[key] = new_entry

        self.access_log_lock.acquire()
        self.access_log.appendleft(new_entry)
        self.access_log_lock.release()