Start a concurrent operation.

        This gets the concurrency limiter for the given key (creating it if
        necessary) and starts a concurrent operation on it.

def start(self, key):
        """
        Start a concurrent operation.

        This gets the concurrency limiter for the given key (creating it if
        necessary) and starts a concurrent operation on it.
        """
        start_d = self._get_limiter(key).start()
        self._cleanup_limiter(key)
        return start_d