store: add waitgroup test.
store: add waitgroup test.

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