fix(lib): keep the server connection alive with TCP keepalive

A NAT that drops an idle mapping leaves the server socket looking
connected while the server can no longer reach us, and nothing errors:
peers stop finding us and searches go unanswered until a restart. The
server socket now carries TCP keepalive (idle 10s, interval 2s, ten
probes), which keeps the mapping alive and turns a dead path into a read
error within about half a minute, as Nicotine+ does. The server stopped
answering ServerPing (code 32) long ago, so an application ping is no
substitute. Windows gets a no-op until the daemon ships there.

soulseek-rs-lib/src/actor/server_actor.rs

@@ -350,6 +350,9 @@ impl ServerActor {
             return false;
         }
         stream.set_nodelay(true).ok();
+        if let Err(e) = crate::utils::keepalive::set_keepalive(&stream) {
+            warn!("[server] keepalive not set: {}", e);
+        }
 
         self.stream = Some(stream);
         self.connection_state = ConnectionState::Connecting {
@@ -1020,6 +1023,22 @@ mod tests {
         );
     }
 
+    #[cfg(unix)]
+    #[test]
+    fn the_server_socket_keeps_itself_alive() {
+        let listener = std::net::TcpListener::bind("127.0.0.1:0").unwrap();
+        let mut actor = parked_actor(listener.local_addr().unwrap().port());
+
+        assert!(actor.initiate_connection());
+
+        assert!(
+            crate::utils::keepalive::keepalive_enabled(
+                actor.stream.as_ref().unwrap()
+            ),
+            "an idle NAT mapping must be kept alive and a dead path noticed"
+        );
+    }
+
     fn parked_actor(port: u16) -> ServerActor {
         let (tx, _rx) = std::sync::mpsc::channel();
         ServerActor::new(