protocol: Decrease log-level for logging ping/pong — heartwood

When logging writes, use a lower more verbose level for ping and pong, as these messages are quite noisy.

     }
     pub fn write(&mut self, remote: &Session, msg: Message) {
         msg.log(log::Level::Debug, &remote.id, Link::Outbound);
         let level = match &msg {
             Message::Ping(_) | Message::Pong { .. } => log::Level::Trace,
             _ => log::Level::Debug,
         };
         msg.log(level, &remote.id, Link::Outbound);
         trace!(target: "service", "Write {:?} to {}", &msg, remote);
         self.io.push_back(Io::Write(remote.id, vec![msg]));