连接断开的频率太高导致报错，可以在每次连接之间sleep，或者保持一个长连接。
ref：https://stackoverflow.com/questions/24884438/2003-cant-connect-to-mysql-server-on-127-0-0-13306-99-cannot-assign-reques
http://www.fromdual.com/huge-amount-of-time-wait-connections
https://vincent.bernat.im/en/blog/2014-tcp-time-wait-state-linux