reorder mysqlConn fields and remove unused protocol field
reorder mysqlConn fields and remove unused protocol field

Fixes #145

springning/mysql,cw2018/mysql,helight/helight_code,segmentio/mysql,Go-SQL-Driver/MySQL,Vorstack/mysql,yyljlyy/mysql,henrylee2cn/mysql,dalinaum/go-mysql-driver,methane/mysql,helight/helight_code,siriele/mysql,aperera/mysql,helight/helight_code,joshuaprunier/mysql,Frca/mysql,bookerzzz/mysql,VividCortex/mysql,beego/mysql,hebihui/mysql,helight/helight_code,chinanjjohn2012/mysql,tmulchay/mysql,helight/helight_code,sunjian1/mysql,Freeaqingme/golang-sql-driver-mysql,helight/helight_code,roth1002/mysql,bookerzzz/mysql,muthusubbu459/mysql,cxflag203/mysql,eklitzke/mysql,go-sql-driver/mysql,wfxiang08/mysql,rlowe/mysql,nanata1115/mysql,eXcomm/gomysql,hyperworks/mysql,zhgo/mysql,jockchou/mysql,rafaeljusto/mysql,LightKool/mysql-go,helight/helight_code,yonglehou/mysql,methane/mysql,LightKool/mysql-go,helight/helight_code,segmentio/mysql,netroby/mysql,VividCortex/mysql,helight/helight_code