Allow hot reload of SSL certificates

[bentoi]

There's no easy way to hot reload certificates with IceSSL right now. With certificate authorities such as Let's Encrypt that delivers short-lived certificates, it would be good to add support for this to allow a process to reload the certificates without having to be restarted.

See also https://forums.zeroc.com/discussion/46665/can-glacier2-reload-ssl-certificates-at-runtime

We should also look into the ACME protocol to renew certificates and see if we can eventually integrate with it to make it easier to renew the certificates (although there are already clients which are doing this such a certbot... so this probably doesn't really belong to Ice).