func (rwm *RWMutex) waitOnLastRev(pfx string) (bool, error) {
	client := rwm.s.Client()
	// get key that's blocking myKey
	opts := append(v3.WithLastRev(), v3.WithMaxModRev(rwm.myKey.Revision()-1))
	lastKey, err := client.Get(rwm.ctx, pfx, opts...)
	if err != nil {
		return false, err
	}
	if len(lastKey.Kvs) == 0 {
		return true, nil
	}
	// wait for release on blocking key
	_, err = WaitEvents(
		client,
		string(lastKey.Kvs[0].Key),
		rwm.myKey.Revision(),
		[]mvccpb.Event_EventType{mvccpb.DELETE})
	return false, err
}
func (rwm *RWMutex) waitOnLastRev(pfx string) (bool, error) {
	client := rwm.s.Client()
	// get key that's blocking myKey
	opts := append(v3.WithLastRev(), v3.WithMaxModRev(rwm.myKey.Revision()-1))
	lastKey, err := client.Get(rwm.ctx, pfx, opts...)
	if err != nil {
		return false, err
	}
	if len(lastKey.Kvs) == 0 {
		return true, nil
	}
	// wait for release on blocking key
	_, err = WaitEvents(
		client,
		string(lastKey.Kvs[0].Key),
		rwm.myKey.Revision(),
		[]mvccpb.Event_EventType{mvccpb.DELETE})
	return false, err
}
// waitOnLowest will wait on the last key with a revision < rwm.myKey.Revision with a
// given prefix. If there are no keys left to wait on, return true.