Store old server SSL configurations

If the SSL configuration of a server was altered successfully, it would
overwrite an existing configuration leading to a true memory
leak. Converting the SSL_LISTENER structure to a list allows it to store
the old configurations without leaking the memory.

This has no functional benefits apart from storing references which could
aid in debugging. In the future, the discarded configurations could be
freed once all connections that use it are closed.

include/maxscale/gw_ssl.h

@@ -71,6 +71,7 @@ typedef struct ssl_listener
     char *ssl_key;                      /*< SSL private key */
     char *ssl_ca_cert;                  /*< SSL CA certificate */
     bool ssl_init_done;                 /*< If SSL has already been initialized for this service */
+    struct ssl_listener *next;          /*< Next SSL configuration, currently used to store obsolete configurations */
 } SSL_LISTENER;
 
 int ssl_authenticate_client(struct dcb *dcb, bool is_capable);