message: qlog: Fix packet length calculation
  builder.len() includes any previous packets' lengths as well. Instead,
  just record the length from the start of the header.
  Also include aead expansion in packet_sent() packet size.
  Extra local variable use was required to avoid borrowck error.
diff: @@ -1623,14 +1623,15 @@ impl Connection {
  let payload_start = builder.len();
  // Work out if we have space left.
  - if builder.len() + tx.expansion() > profile.limit() {
  + let aead_expansion = tx.expansion();
  + if builder.len() + aead_expansion > profile.limit() {
  // No space for a packet of this type.
  encoder = builder.abort();
  continue;
  }
  // Add frames to the packet.
  - let limit = profile.limit() - tx.expansion();
  + let limit = profile.limit() - aead_expansion;
  let (tokens, ack_eliciting) =
  self.add_frames(&mut builder, *space, limit, &profile, now);
  if builder.is_empty() {
  @@ -1644,7 +1645,7 @@ impl Connection {
  &mut self.qlog,
  pt,
  pn,
  - builder.len(),
  + builder.len() - header_start + aead_expansion,
  &builder[payload_start..],
  );
  