mqtt自动清除会话

[github9157]

文件：mqtt_client.c 代码：if (cid_len == 0) cli->clean_session = 1;
应改成：if (cid_len > 0) cli->clean_session = 1; 现在未开放clean_session接口；不设置的话一直不自动清会话； 临时设置进行clean_session if(m_ptrMqttClient->client) { m_ptrMqttClient->client->clean_session = 1; }

[ithewei]

clean_session选项就是需要自己设置的，不适合在内部写死cli->clean_session = 1