Further adapt prototypes of ticket functions

Moving everything in ticket_keys structure, that will soon become
ticket_context.
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 7adaaff..25a4deb 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -5704,7 +5704,7 @@
 
 void mbedtls_ssl_conf_session_ticket_lifetime( mbedtls_ssl_config *conf, int lifetime )
 {
-    conf->ticket_lifetime = lifetime;
+    conf->ticket_keys->ticket_lifetime = lifetime;
 }
 #endif /* MBEDTLS_SSL_SESSION_TICKETS */
 
@@ -6700,7 +6700,8 @@
 #endif
 
 #if defined(MBEDTLS_SSL_SESSION_TICKETS)
-    conf->ticket_lifetime = MBEDTLS_SSL_DEFAULT_TICKET_LIFETIME;
+    ssl_ticket_keys_init( &conf );
+    conf->ticket_keys->ticket_lifetime = MBEDTLS_SSL_DEFAULT_TICKET_LIFETIME;
 #endif
 
 #if defined(MBEDTLS_SSL_SET_CURVES)