optimized buffer refill
optimized buffer refill

No extra allocation when refilling the buffer
Closes #52

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