raft: add test case for leader transfer from follower
raft: add test case for leader transfer from follower

endocode/etcd,vimalk78/etcd,xiang90/etcd,hongchaodeng/etcd,lorneli/etcd,etcd-io/etcd,fanminshi/etcd,dannysauer/etcd,mkumatag/etcd,mqliang/etcd,ravigadde/etcd,plasticbox/etcd,joshix/etcd,jlamillan/etcd,lumjjb/etcd,yudai/etcd,sebrandon1/etcd,qqsun8819/etcd,vimalk78/etcd,armstrongli/etcd,rnaveiras/etcd,ravigadde/etcd,handsomesun/etcd,mkumatag/etcd,insequent/etcd,armstrongli/etcd,glevand/coreos--etcd,qqsun8819/etcd,sebrandon1/etcd,luxas/etcd,insequent/etcd,yudai/etcd,heyitsanthony/etcd,visheshnp/etcd,handsomesun/etcd,etcd-io/etcd,tguangch/etcd,a-robinson/etcd,sinsharat/etcd,cockroachdb/etcd,bdarnell/etcd,jlamillan/etcd,plasticbox/etcd,ajityagaty/etcd,fanminshi/etcd,glevand/coreos--etcd,endocode/etcd,dannysauer/etcd,coreos/etcd,etcd-io/etcd,mkumatag/etcd,sinsharat/etcd,luxas/etcd,ajityagaty/etcd,bluebreezecf/etcd,rchicoli/etcd,bdarnell/etcd,YuleiXiao/etcd,zaphod-concur/etcd,tguangch/etcd,akauppi/etcd,nekto0n/etcd,OyTao/etcd_learning,gyuho/etcd,rchicoli/etcd,xiang90/etcd,YuleiXiao/etcd,li-ang/etcd,visheshnp/etcd,lumjjb/etcd,li-ang/etcd,whuwxl/etcd,rnaveiras/etcd,CrazyJvm/etcd,joshix/etcd,gyuho/etcd,hongchaodeng/etcd,rnaveiras/etcd,zaphod-concur/etcd,nekto0n/etcd,OyTao/etcd_learning,coreos/etcd,heyitsanthony/etcd,akauppi/etcd,a-robinson/etcd,mickep76/etcd,mqliang/etcd,cockroachdb/etcd,mqliang/etcd,ajityagaty/etcd,lorneli/etcd,CrazyJvm/etcd,whuwxl/etcd,mickep76/etcd,bluebreezecf/etcd