Drop a guest's GamesServ games on disconnect so a recycled uid can't inherit one

api/src/lib.rs

@@ -2463,6 +2463,11 @@ pub trait Service: Send {
     fn help_topics(&self) -> (&'static str, &'static [HelpEntry]) {
         ("", &[])
     }
+    // A user left the network (QUIT / KILL / netsplit). Lets a service drop any
+    // per-connection state it keyed on that departing uid before the uid is recycled
+    // to a new connection. Default: nothing to do.
+    fn on_user_quit(&mut self, _uid: &str) {}
+
     fn on_command(&mut self, from: &Sender, args: &[&str], ctx: &mut ServiceCtx, net: &dyn NetView, store: &mut dyn Store);
 }