Handles future salt results, which don't come inside a
        ``rpc_result`` but are still sent through a request:

            future_salts#ae500895 req_msg_id:long now:int
            salts:vector<future_salt> = FutureSalts;

async def _handle_future_salts(self, message):
        """
        Handles future salt results, which don't come inside a
        ``rpc_result`` but are still sent through a request:

            future_salts#ae500895 req_msg_id:long now:int
            salts:vector<future_salt> = FutureSalts;
        """
        # TODO save these salts and automatically adjust to the
        # correct one whenever the salt in use expires.
        self._log.debug('Handling future salts for message %d', message.msg_id)
        state = self._pending_state.pop(message.msg_id, None)
        if state:
            state.future.set_result(message.obj)