code: func (s *HybridSlowStart) OnPacketAcked(ackedPacketNumber protocol.PacketNumber) {
  	if s.IsEndOfRound(ackedPacketNumber) {
  		s.started = false
  	}
  }
docs: // OnPacketAcked gets invoked after ShouldExitSlowStart, so it's best to end
  // the round when the final packet of the burst is received and start it on
  // the next incoming ack.