func prevKeyFromPuts(resp *clientv3.TxnResponse) *mvccpb.KeyValue {
	for _, r := range resp.Responses {
		pkv := r.GetResponsePut().PrevKv
		if pkv != nil && pkv.CreateRevision > 0 {
			return pkv
		}
	}
	return nil
}
func prevKeyFromPuts(resp *clientv3.TxnResponse) *mvccpb.KeyValue {
	for _, r := range resp.Responses {
		pkv := r.GetResponsePut().PrevKv
		if pkv != nil && pkv.CreateRevision > 0 {
			return pkv
		}
	}
	return nil
}
// prevKeyFromPuts gets the prev key that is being put; ignores
// the put action response.