server: scrub a departed user's pending invites via a User.invited reverse index instead of scanning every channel on the network per quit — the old O(channels)-per-quit path was O(channels*quits) on a netsplit; the index is maintained at the 4 invite add/remove sites (INVITE cmd, S2S INVITE, join-consume, UNINVITE)

src/channels.rs

@@ -907,6 +907,7 @@ impl Server {
         ch.recent_kicks.remove(&uid); // they got back in; clear any +J rejoin timer
         if let Some(u) = self.users.get_mut(&uid) {
             u.channels.insert(key.clone());
+            u.invited.remove(&key); // invite consumed — drop it from the reverse index
         }
         // chancreate: snotice when a brand-new channel comes into being
         if is_new