Enhance signal handling to support proper daemon operation:
- SIGTERM: Graceful shutdown (same behavior as SIGINT)
- SIGQUIT: Graceful shutdown with full statistics dump
- SIGUSR1: Log rotation acknowledgment for external tools
- SIGUSR2: Dump runtime status to log without stopping
Statistics dump includes connection counts, ME keepalive metrics,
and relay adaptive tuning counters. SIGHUP config reload unchanged
(handled in hot_reload.rs).
Signals are handled via tokio::signal::unix with async select!
to avoid blocking the runtime. Non-shutdown signals (USR1/USR2)
run in a background task spawned at startup.
Signed-off-by: Vladimir Krivopalov <argenet@yandex.ru>
Vladimir Krivopalov