Adds the given message to the list of messages that must be
        acknowledged and dispatches control to different ``_handle_*``
        method based on its type.

async def _process_message(self, message):
        """
        Adds the given message to the list of messages that must be
        acknowledged and dispatches control to different ``_handle_*``
        method based on its type.
        """
        self._pending_ack.add(message.msg_id)
        handler = self._handlers.get(message.obj.CONSTRUCTOR_ID,
                                     self._handle_update)
        await handler(message)