OperServ: per-IP session limiting with exceptions

The connection that puts an IP over its allowance is killed on connect.
The ircd's per-user IP (UID param 7) is now plumbed through UserConnect and
tracked per-IP in the live network view (incremented on connect, released
on quit).

- [session] default_limit sets the base allowance (0/absent = off).
- OperServ SESSION LIST <min> / VIEW <ip> inspect live counts.
- OperServ EXCEPTION ADD <ip-mask> <limit> [reason] / DEL / LIST override
  the limit per IP-mask (limit 0 = unlimited); event-sourced so they
  federate and survive restart, matched most-permissive-wins.

All admin-gated.

src/main.rs

@@ -115,6 +115,9 @@ async fn main() -> Result<()> {
     if let Some(expire) = &cfg.expire {
         engine.lock().await.set_expiry(expire.account_ttl(), expire.channel_ttl(), expire.warn_ttl());
     }
+    if let Some(session) = &cfg.session {
+        engine.lock().await.set_session_limit(session.limit());
+    }
 
     if let Some(gossip) = cfg.gossip.clone() {
         tracing::info!(peers = cfg.peer.len(), "starting gossip");