clientv3: return copy of endpoints, not pointer
clientv3: return copy of endpoints, not pointer

Fix https://github.com/coreos/etcd/issues/6892.

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