message: Make Connection::pmtu a method
  It can vary based on ipv4 or ipv6, or if we ever implemenet PMTUD
  fixes
diff: @@ -306,7 +306,6 @@ pub struct Connection {
  connection_ids: HashMap<u64, (Vec<u8>, [u8; 16])>, // (sequence number, (connection id, reset token))
  pub(crate) send_streams: SendStreams,
  pub(crate) recv_streams: RecvStreams,
  - pmtu: usize,
  pub(crate) flow_mgr: Rc<RefCell<FlowMgr>>,
  loss_recovery: LossRecovery,
  loss_recovery_state: LossRecoveryState,
  @@ -422,7 +421,6 @@ impl Connection {
  connection_ids: HashMap::new(),
  send_streams: SendStreams::default(),
  recv_streams: RecvStreams::default(),
  - pmtu: 1280,
  flow_mgr: Rc::new(RefCell::new(FlowMgr::default())),
  loss_recovery: LossRecovery::new(),
  loss_recovery_state: LossRecoveryState::default(),
  @@ -437,6 +435,14 @@ impl Connection {
  self.tps.borrow_mut().local.set(key, value)
  }
  + fn pmtu(&self) -> usize {
  + match &self.paths {
  + Some(path) if path.local.is_ipv4() => 1252,
  + Some(_) => 1232, // IPv6
  + None => 1280,
  + }
  + }
  +
  /// Set the connection ID that was originally chosen by the client.
  pub(crate) fn original_connection_id(&mut self, odcid: &ConnectionId) {
  assert_eq!(self.role, Role::Server);
  @@ -1006,7 +1012,7 @@ impl Connection {
  match &self.state {
  State::Init | State::WaitInitial | State::Handshaking | State::Connected => {
  loop {
  - let remaining = self.pmtu - out_bytes.len() - encoder.len();
  + let remaining = self.pmtu() - out_bytes.len() - encoder.len();
  // Check sources in turn for available frames
  if let Some((frame, token)) = self
  @@ -1024,8 +1030,8 @@ impl Connection {
  if let Some(t) = token {
  tokens.push(t);
  }
  - assert!(encoder.len() <= self.pmtu);
  - if out_bytes.len() + encoder.len() == self.pmtu {
  + assert!(encoder.len() <= self.pmtu());
  + if out_bytes.len() + encoder.len() == self.pmtu() {
  // No more space for frames.
  break;
  }
  @@ -1111,7 +1117,7 @@ impl Connection {
  let mut packet = encode_packet(tx, &hdr, &encoder);
  dump_packet(self, "TX ->", &hdr, &encoder);
  out_bytes.append(&mut packet);
  - if out_bytes.len() >= self.pmtu {
  + if out_bytes.len() >= self.pmtu() {
  break;
  }
  }
  