StartTLSProtocol: Changed ssl_keys to contain names of key pairs. This changes ssl_keys from being a set of keypair identifier numbers to a set of keypair names. This should make updating certificates on the fly (with eg Let's Encrypt) behave more like users expect. Previously if the certificate switched to a new key it was regarded as a new certificate and manual configuration was needed. Now the new certificate and key will most likely have the same name, and will thus be used automatically. Fixes [WS-580].