Stop a concurrent operation.

        This gets the concurrency limiter for the given key (creating it if
        necessary) and stops a concurrent operation on it. If the concurrency
        limiter is empty, it is deleted.

def stop(self, key):
        """
        Stop a concurrent operation.

        This gets the concurrency limiter for the given key (creating it if
        necessary) and stops a concurrent operation on it. If the concurrency
        limiter is empty, it is deleted.
        """
        self._get_limiter(key).stop()
        self._cleanup_limiter(key)