修复MongoDB连接bug
修复MongoDB连接bug

修复在MongoDB服务端异常情况下，启动vnpy依然显示MongoDB连接成功bug。
修改‘serverSelectionTimeoutMS’为3s(默认30s), 利用server_info()函数测试服务器是否正常。
andrewchenshx/vnpy,yongfuyang/vnpy,kernelmilowill/PDMQBACKTEST,Chemcy/vnpy,freeitaly/Trading-System,kernelmilowill/PDMQBACKTEST,vnpy/vnpy,andrewchenshx/vnpy,jiangjinjinyxt/vnpy,rrrrrr8/vnpy,wylwang/vnpy,msincenselee/vnpy,bigdig/vnpy,ujfjhz/vnpy,msincenselee/vnpy,ujfjhz/vnpy,andrewchenshx/vnpy,harveywwu/vnpy,kernelmilowill/PDMQBACKTEST,wylwang/vnpy,msincenselee/vnpy,wisfern/vnpy,rrrrrr8/vnpy,xcbat/vnpy,kernelmilowill/PDMQBACKTEST,vnpy/vnpy,rrrrrr8/vnpy,akeyong/vnpy,xcbat/vnpy,yongfuyang/vnpy,yongfuyang/vnpy,msincenselee/vnpy,bigdig/vnpy,harveywwu/vnpy,xcbat/vnpy,akeyong/vnpy,wylwang/vnpy,Chemcy/vnpy,freeitaly/Trading-System,andrewchenshx/vnpy,freeitaly/Trading-System,jiangjinjinyxt/vnpy,harveywwu/vnpy,lukesummer/vnpy,xcbat/vnpy,lukesummer/vnpy,rrrrrr8/vnpy,Chemcy/vnpy,freeitaly/Trading-System,ujfjhz/vnpy,kernelmilowill/PDMQBACKTEST,akeyong/vnpy,Chemcy/vnpy,xcbat/vnpy,harveywwu/vnpy,wisfern/vnpy,wisfern/vnpy,wylwang/vnpy,lukesummer/vnpy,wisfern/vnpy,jiangjinjinyxt/vnpy,jiangjinjinyxt/vnpy,ujfjhz/vnpy,lukesummer/vnpy,lukesummer/vnpy,harveywwu/vnpy,freeitaly/Trading-System,jiangjinjinyxt/vnpy,wylwang/vnpy,rrrrrr8/vnpy,rrrrrr8/vnpy,wisfern/vnpy,yongfuyang/vnpy,harveywwu/vnpy,ujfjhz/vnpy,andrewchenshx/vnpy,yongfuyang/vnpy,Chemcy/vnpy,bigdig/vnpy,akeyong/vnpy,akeyong/vnpy,bigdig/vnpy,wisfern/vnpy