Isolate listeners per server

[mattn]

Move the global listeners map and mutex onto Server so each Server instance manages its own subscriptions, eliminating cross-server interference; GetListeningFilters and BroadcastEvent now iterate registered servers.

Benefits:

1. Servers no longer interfere with each other — multiple relays can coexist in one process without subscriptions leaking across them.
2. Lock contention is reduced — each Server has its own mutex, improving concurrency.
3. No state leaks — discarding a Server also discards its listeners.
4. Tests are independent — without global state, parallel and sequential tests don't interfere.