[ImgBot] Optimize images

*Total -- 58.20kb -> 26.53kb (54.42%)

/images/dubbo-not-keep-connection.png -- 11.80kb -> 4.77kb (59.59%)
/docs/distributed-system/images/dubbo-not-keep-connection.png -- 11.80kb -> 4.77kb (59.59%)
/images/dubbo-keep-connection.png -- 17.30kb -> 8.49kb (50.9%)
/docs/distributed-system/images/dubbo-keep-connection.png -- 17.30kb -> 8.49kb (50.9%)