tidb-server/server: fix shadow found by Go 1.6 vet.
tidb-server/server: fix shadow found by Go 1.6 vet.

liyu4/tidb,kyle-wang/tidb,zimulala/tidb,Nify/tidb,EvilMcJerkface/tidb,wusphinx/tidb,silentred/tidb,kyle-wang/tidb,ngaut/tidb,ming-relax/tidb,8cbx/tidb,pingcap/tidb,pingcap/tidb,andelf/tidb,c4pt0r/tidb,pingcap/tidb,qhsong/tidb,c4pt0r/tidb,tiancaiamao/tidb,anywhy/tidb,myrice/tidb,CodeRushing/tidb,Nify/tidb,andelf/tidb,silentred/tidb,ngaut/tidb,mccxj/tidb,c4pt0r/tidb,pingcap/tidb,kyle-wang/tidb,Cofyc/tidb,cosmtrek/tidb,qiuyesuifeng/tidb,c4pt0r/tidb,simon-xia/tidb,8cbx/tidb,pingcap/tidb,JinheLin/tidb,yangwenmai/tidb,simon-xia/tidb,yuanwhy/tidb,spongedu/tidb,XuHuaiyu/tidb,liyu4/tidb,astaxie/tidb,ngaut/tidb,CodeRushing/tidb,wusphinx/tidb,mccxj/tidb,gaocegege/tidb,yangwenmai/tidb,silenceper/tidb,c4pt0r/tidb,kyle-wang/tidb,gaocegege/tidb,louishust/tidb,tiancaiamao/tidb,Cofyc/tidb,silenceper/tidb,zimulala/tidb,anywhy/tidb,ueizhou/tidb,ngaut/tidb,ngaut/tidb,spongedu/tidb,c4pt0r/tidb,EvilMcJerkface/tidb,JinheLin/tidb,qiuyesuifeng/tidb,ngaut/tidb,ming-relax/tidb,qhsong/tidb,coocood/tidb,ueizhou/tidb,astaxie/tidb,kyle-wang/tidb,kyle-wang/tidb,zs634134578/tidb,myrice/tidb,cosmtrek/tidb,XuHuaiyu/tidb,zs634134578/tidb,louishust/tidb,coocood/tidb,yuanwhy/tidb,pingcap/tidb