message: PTO packets should still be tracked but are not in-flight
  i.e. we still need to call on_packet_sent() for them.
diff: @@ -1242,22 +1242,20 @@ impl Connection {
  let mut packet = encode_packet(tx, &hdr, &encoder);
  - if self.tx_mode != TxMode::Pto {
  - if ack_eliciting {
  + if self.tx_mode != TxMode::Pto && ack_eliciting {
  self.idle_timeout.on_packet_sent(now);
  }
  +
  + let in_flight = match self.tx_mode {
  + TxMode::Pto => false,
  + TxMode::Normal => ack_eliciting || has_padding,
  + };
  +
  self.loss_recovery.on_packet_sent(
  space,
  hdr.pn,
  - SentPacket::new(
  - now,
  - ack_eliciting,
  - tokens,
  - packet.len(),
  - ack_eliciting || has_padding,
  - ),
  + SentPacket::new(now, ack_eliciting, tokens, packet.len(), in_flight),
  );
  - }
  dump_packet(self, "TX ->", &hdr, &encoder);
  