add system settings

broker/migrate/mysql/V18__create_settings_table.sql

@@ -0,0 +1,13 @@
+
+DROP TABLE IF EXISTS `t_settings`;
+CREATE TABLE `t_settings` (
+  `id` int(11) NOT NULL PRIMARY KEY,
+  `value` varchar(64) NOT NULL,
+  `desc` varchar(128) NOT NULL default ""
+)
+ENGINE = InnoDB
+DEFAULT CHARACTER SET = utf8mb4
+COLLATE = utf8mb4_unicode_ci;
+
+
+insert into t_settings(`id`, `value`,`desc`) values (1, "3000", "最大群成员数量");