Replace the fixed 30s sleep-based shutdown drain with actual in-flight RPC
tracking using an Arc<AtomicUsize> counter and RAII InFlightGuard. On
SIGTERM/SIGINT the server now:
1. Stops accepting new client and federation connections
2. Sends QUIC CONNECTION_CLOSE with reason "server shutting down"
3. Polls the in-flight counter until it reaches 0 (or drain timeout)
4. Logs drain progress as RPCs complete
5. Calls plugin on_shutdown hooks before exit
Also adds:
- on_shutdown hook to HookVTable (C-ABI plugin API) and ServerHooks trait
- server_in_flight_rpcs Prometheus gauge metric
- Federation connection tracking via shared in-flight counter
Christian Nennemann
66eca065e0