optimize

doc/db/tables_xxl_job.sql

   `registry_value` varchar(255) NOT NULL,
   `update_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
   PRIMARY KEY (`id`),
-  KEY `i_g_k_v` (`registry_group`,`registry_key`,`registry_value`)
+  KEY `i_g_k_v` (`registry_group`,`registry_key`,`registry_value`),
+  KEY `i_u` (`update_time`)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
 
 CREATE TABLE `xxl_job_group` (