etcdserver: remove a data race of ServerStat
etcdserver: remove a data race of ServerStat

It seems that ServerStats.BecomeLeader() is missing a lock.

Fix https://github.com/coreos/etcd/issues/5155

wantonsolutions/dinv-etcd,ajityagaty/etcd,achanda/etcd,gyuho/etcd,chipironcin/etcd,ajityagaty/etcd,bdarnell/etcd,YuleiXiao/etcd,dnaeon/etcd,lorneli/etcd,OyTao/etcd_learning,etcd-io/etcd,luxas/etcd,gyuho/etcd,tguangch/etcd,YuleiXiao/etcd,zaphod-concur/etcd,yudai/etcd,visheshnp/etcd,endocode/etcd,dannysauer/etcd,ravigadde/etcd,akauppi/etcd,a-robinson/etcd,mqliang/etcd,fanminshi/etcd,qqsun8819/etcd,yudai/etcd,a-robinson/etcd,linuxcer/etcd,armstrongli/etcd,nekto0n/etcd,jonboulle/etcd,whuwxl/etcd,wantonsolutions/dinv-etcd,westhood/etcd,visheshnp/etcd,handsomesun/etcd,mqliang/etcd,jonboulle/etcd,armstrongli/etcd,joshix/etcd,glycerine/etcd,bdarnell/etcd,tguangch/etcd,bluebreezecf/etcd,mickep76/etcd,cockroachdb/etcd,luxas/etcd,mqliang/etcd,plasticbox/etcd,CrazyJvm/etcd,zaphod-concur/etcd,heyitsanthony/etcd,rchicoli/etcd,ravigadde/etcd,mkumatag/etcd,fanminshi/etcd,coreos/etcd,handsomesun/etcd,mkumatag/etcd,glevand/coreos--etcd,wantonsolutions/dinv-etcd,etcd-io/etcd,linuxcer/etcd,swingbach/etcd,hongchaodeng/etcd,bluebreezecf/etcd,ktateish/etcd,sebrandon1/etcd,li-ang/etcd,dnaeon/etcd,rnaveiras/etcd,rnaveiras/etcd,chipironcin/etcd,dannysauer/etcd,endocode/etcd,jlamillan/etcd,nekto0n/etcd,sofuture/etcd,sofuture/etcd,rchicoli/etcd,lumjjb/etcd,StorReduce/etcd,heyitsanthony/etcd,westhood/etcd,sebrandon1/etcd,achanda/etcd,joshix/etcd,insequent/etcd,etcd-io/etcd,glycerine/etcd,xiang90/etcd,mkumatag/etcd,lorneli/etcd,StorReduce/etcd,hongchaodeng/etcd,sinsharat/etcd,ktateish/etcd,lumjjb/etcd,plasticbox/etcd,vimalk78/etcd,insequent/etcd,mickep76/etcd,qqsun8819/etcd,CrazyJvm/etcd,glevand/coreos--etcd,ajityagaty/etcd,purpleidea/etcd,rnaveiras/etcd,swingbach/etcd,li-ang/etcd,OyTao/etcd_learning,akauppi/etcd,cockroachdb/etcd,whuwxl/etcd,xiang90/etcd,vimalk78/etcd,purpleidea/etcd,coreos/etcd,sinsharat/etcd,jlamillan/etcd