Rotate SSL certificates on reload (SIGHUP) without restart

Hello,

Please consider adding support for rotating SSL certificates on reloading
pgpool2 (i.e., sending SIGHUP to the pgpool parent), so that certificate
rotations do not require a full service restart. PostgreSQL can pick up new
certificates on reload/SIGHUP; pgpool currently requires a restart, which
causes connection disruptions.

*Current behavior:*

- Replace certificate/key files used by pgpool (e.g., server.crt,
server.key, related CA chain).
- Run systemctl reload pgpool2 (send SIGHUP to the pgpool parent).
- Observations: Existing and new client connections continue to present
the old certificate. Only systemctl restart pgpool2 applies the new certs
(causing connection interruptions).

*Expected behavior:*

- After systemctl reload pgpool2 / SIGHUP, pgpool should re-read
SSL-related configuration (server cert, private key, chain/CA, CRL if
configured) and use them for new client connections, without requiring a
full restart.
- Existing connections can continue with the old context; only new
handshakes should use the updated materials.
- If reload fails, log a clear error and keep using the previous context
to avoid breaking clients.
- Consider parity with PostgreSQL’s SIGHUP behavior for certificate
reloads where feasible.

Regards,
Bob Ross