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/grpc.rs

@@ -174,7 +174,9 @@ fn to_wire(entry: &LogEntry) -> Option<ReplicationEvent> {
         | Event::NewsAdded { .. }
         | Event::NewsDeleted { .. }
         | Event::OperGranted { .. }
-        | Event::OperRevoked { .. } => return None,
+        | Event::OperRevoked { .. }
+        | Event::SessionExceptionAdded { .. }
+        | Event::SessionExceptionRemoved { .. } => return None,
     };
     Some(ReplicationEvent { origin: entry.origin().to_string(), seq: entry.seq(), lamport: entry.lamport(), kind: Some(kind) })
 }