client/v3/maintenance.go: Add Downgrade support to client
client/v3/maintenance.go: Add Downgrade support to client

mkumatag/etcd,rnaveiras/etcd,etcd-io/etcd,mqliang/etcd,rnaveiras/etcd,ajityagaty/etcd,gyuho/etcd,etcd-io/etcd,mkumatag/etcd,coreos/etcd,gyuho/etcd,etcd-io/etcd,mkumatag/etcd,rnaveiras/etcd,mqliang/etcd,ajityagaty/etcd,mqliang/etcd,coreos/etcd,ajityagaty/etcd