Log more information during an idle timeout

Show current state, handle the case when there were dual FIN's and an
ACK got dropped (doesn't need a scary WARN log).

src/conn.rs

@@ -306,8 +306,24 @@ impl<const N: usize, P: ConnectionPeer> Connection<N, P> {
                 }
                 () = &mut idle_timeout => {
                     if !std::matches!(self.state, State::Closed { .. }) {
+                        // Warn that we are giving up on the connection due to a lack of activity.
+
+                        // First, we look for this special situation:
+                        // Our outgoing FIN was sent, but the incoming ACK was dropped.
+                        // This timeout is not worthy of a warning log, because everything
+                        // basically went fine: all data was sent and ACK'd.
+                        // There is no more work to do.
+
                         let unacked: Vec<u16> = self.unacked.keys().copied().collect();
-                        tracing::warn!(?unacked, "idle timeout expired, closing...");
+                        if let State::Closing { local_fin, remote_fin, .. } = self.state {
+                            if unacked.len() == 1 && local_fin.is_some() && &local_fin.unwrap() == unacked.last().unwrap() && remote_fin.is_some() {
+                                tracing::debug!(?self.state, ?unacked, "Idle timeout with both FINs sent, with only FIN unacked.");
+                            } else {
+                                tracing::warn!(?self.state, ?unacked, "quitting idle connection...");
+                            }
+                        } else {
+                            tracing::warn!(?self.state, ?unacked, "quitting idle connection...");
+                        }
 
                         self.state = State::Closed { err: Some(Error::TimedOut) };
                     }