fix mysql broken pipe
fix mysql broken pipe

去掉长时间不使用的mysql连接，防止mysq的server端wait_timeout后 造成连接不可用、进而产生broken pipe的错误日志
Cepave/open-falcon-backend,taomaree/falcon-plus,jojohappy/falcon-plus,masato25/open-falcon-backend,jojohappy/falcon-plus,masato25/open-falcon-backend,open-falcon/falcon-plus,Cepave/open-falcon-backend,open-falcon/falcon-plus,open-falcon/graph,jojohappy/falcon-plus,yubo/graph,open-falcon/graph,taomaree/falcon-plus,open-falcon/falcon-plus,Cepave/open-falcon-backend,open-falcon/falcon-plus,masato25/open-falcon-backend,masato25/open-falcon-backend,yubo/graph,taomaree/falcon-plus,open-falcon/falcon-plus,Cepave/open-falcon-backend,taomaree/falcon-plus,Cepave/open-falcon-backend,taomaree/falcon-plus,jojohappy/falcon-plus,jojohappy/falcon-plus,masato25/open-falcon-backend,jojohappy/falcon-plus,masato25/open-falcon-backend,taomaree/falcon-plus,Cepave/open-falcon-backend