MqttClient 字段

对客户端而言是的通讯用的套接字，对服务器来说是用于侦听的套接字
A communication socket for the client, or a listening socket for the server

文件传输的时候的缓存大小，直接影响传输的速度，值越大，传输速度越快，越占内存，默认为100K大小
The size of the cache during file transfer directly affects the speed of the transfer. The larger the value, the faster the transfer speed and the more memory it takes. The default size is 100K.