message: Update process_loop_old to call process_output() repeatedly
  This lets HTTP 0.9 QUIC connections time out, just like H3 connections.
diff: @@ -500,7 +500,7 @@ mod old {
  use neqo_common::{matches, Datagram};
  use neqo_crypto::AuthenticationStatus;
  use neqo_transport::{
  - Connection, ConnectionEvent, FixedConnectionIdManager, State, StreamType,
  + Connection, ConnectionEvent, FixedConnectionIdManager, Output, State, StreamType,
  };
  use super::{emit_datagram, get_output_file, Args};
  @@ -595,10 +595,24 @@ mod old {
  return Ok(client.state().clone());
  }
  - let exiting = !handler.handle(args, client)?;
  + let mut exiting = !handler.handle(args, client)?;
  - let out_dgram = client.process_output(Instant::now());
  - emit_datagram(&socket, out_dgram.dgram());
  + loop {
  + let output = client.process_output(Instant::now());
  + match output {
  + Output::Datagram(dgram) => emit_datagram(&socket, Some(dgram)),
  + Output::Callback(duration) => {
  + socket.set_read_timeout(Some(duration)).unwrap();
  + break;
  + }
  + Output::None => {
  + // Not strictly necessary, since we're about to exit
  + socket.set_read_timeout(None).unwrap();
  + exiting = true;
  + break;
  + }
  + }
  + }
  if exiting {
  return Ok(client.state().clone());
  