store/tikv: export DoGC method of GCWorker for testing purpose. (#1902)
store/tikv: export DoGC method of GCWorker for testing purpose. (#1902)

So we can control when to trigger GC during performance test.
EvilMcJerkface/tidb,pingcap/tidb,ming-relax/tidb,kyle-wang/tidb,gaocegege/tidb,kyle-wang/tidb,c4pt0r/tidb,8cbx/tidb,tiancaiamao/tidb,louishust/tidb,wusphinx/tidb,qhsong/tidb,c4pt0r/tidb,ngaut/tidb,andelf/tidb,zs634134578/tidb,silenceper/tidb,qiuyesuifeng/tidb,pingcap/tidb,ueizhou/tidb,Nify/tidb,qhsong/tidb,ming-relax/tidb,pingcap/tidb,simon-xia/tidb,c4pt0r/tidb,c4pt0r/tidb,yangwenmai/tidb,EvilMcJerkface/tidb,Cofyc/tidb,liyu4/tidb,kyle-wang/tidb,zs634134578/tidb,XuHuaiyu/tidb,ngaut/tidb,astaxie/tidb,gaocegege/tidb,mccxj/tidb,andelf/tidb,simon-xia/tidb,anywhy/tidb,kyle-wang/tidb,ngaut/tidb,mccxj/tidb,qiuyesuifeng/tidb,silentred/tidb,silentred/tidb,pingcap/tidb,wusphinx/tidb,CodeRushing/tidb,cosmtrek/tidb,coocood/tidb,zimulala/tidb,8cbx/tidb,coocood/tidb,pingcap/tidb,CodeRushing/tidb,yangwenmai/tidb,kyle-wang/tidb,ngaut/tidb,myrice/tidb,astaxie/tidb,liyu4/tidb,spongedu/tidb,Nify/tidb,anywhy/tidb,yuanwhy/tidb,c4pt0r/tidb,tiancaiamao/tidb,JinheLin/tidb,louishust/tidb,cosmtrek/tidb,Cofyc/tidb,myrice/tidb,kyle-wang/tidb,spongedu/tidb,yuanwhy/tidb,ueizhou/tidb,c4pt0r/tidb,ngaut/tidb,XuHuaiyu/tidb,JinheLin/tidb,silenceper/tidb,ngaut/tidb,pingcap/tidb,zimulala/tidb