code: func (s *session) closeForRecreating() protocol.PacketNumber {
  	s.destroy(errCloseForRecreating)
  	nextPN, _ := s.sentPacketHandler.PeekPacketNumber(protocol.EncryptionInitial)
  	return nextPN
  }
docs: // closeForRecreating closes the session in order to recreate it immediately afterwards
  // It returns the first packet number that should be used in the new session.