code: func (m *Memberlist) invokeAckHandler(ack ackResp, timestamp time.Time) {
  	m.ackLock.Lock()
  	ah, ok := m.ackHandlers[ack.SeqNo]
  	delete(m.ackHandlers, ack.SeqNo)
  	m.ackLock.Unlock()
  	if !ok {
  		return
  	}
  	ah.timer.Stop()
  	ah.ackFn(ack.Payload, timestamp)
  }
docs: // Invokes an ack handler if any is associated, and reaps the handler immediately