MXS-2794: Log changes in loaded users

By checking whether the users have changed whenever they are reloaded, we
improve the visibility of the user reloading process. Using a checksum
allows us to easily compress the information with acceptable loss of
accuracy. Using a CAS loop prevents duplicate messages without losing any
updates even if multiple user reloads result in different outcomes.

server/modules/authenticator/MariaDBAuth/mysql_auth.hh

@@ -112,6 +112,7 @@ typedef struct mysql_auth
     bool      skip_auth;            /**< Authentication will always be successful */
     bool      check_permissions;
     bool      lower_case_table_names;   /**< Disable database case-sensitivity */
+    uint64_t  checksum;
 } MYSQL_AUTH;
 
 /**