Adds a send_last_message function to the Dispatcher's
        dictionary of functions indexed by connection.

        Args:
            connection (str): A locally unique identifier
                provided by the receiver of messages.
            send_last_message (fn): The method that should be called
                by the dispatcher to respond to messages which
                arrive via connection, when the connection should be closed
                after the message has been sent.

def add_send_last_message(self, connection, send_last_message):
        """Adds a send_last_message function to the Dispatcher's
        dictionary of functions indexed by connection.

        Args:
            connection (str): A locally unique identifier
                provided by the receiver of messages.
            send_last_message (fn): The method that should be called
                by the dispatcher to respond to messages which
                arrive via connection, when the connection should be closed
                after the message has been sent.
        """
        self._send_last_message[connection] = send_last_message
        LOGGER.debug("Added send_last_message function "
                     "for connection %s", connection)