Will the data in the DDL_JOBS table of TiDB be deleted periodically?

### What problem does this PR solve? Currently, we can view the history of DDL jobs and DDL jobs by 'admin show DDL jobs', but it is not convenient to use queries such as group by, so we need to add a system table of DDL_JOBS. ### What is changed and how it works? ``` mysql> select * from information_schema.DDL_JOBS limit 10; +--------+---------------+------------+---------------+--------------+-----------+----------+-----------+-----------------------------------+-----------------------------------+--------+--------------------------------------------------------------------------------------------------------------------------------------------------------+ | JOB_ID | DB_NAME | TABLE_NAME | JOB_TYPE | SCHEMA_STATE | SCHEMA_ID | TABLE_ID | ROW_COUNT | START_TIME | END_TIME | STATE | QUERY | +--------+---------------+------------+---------------+--------------+-----------+----------+-----------+-----------------------------------+-----------------------------------+--------+--------------------------------------------------------------------------------------------------------------------------------------------------------+ | 1718 | test_ddl_jobs | | create schema | public | 1717 | 0 | 0 | 2020-02-18 20:35:40.43 +0800 CST | 2020-02-18 20:35:40.434 +0800 CST | synced | create database if not exists test_ddl_jobs | | 1716 | test_ddl_jobs | | drop schema | none | 1714 | 0 | 0 | 2020-02-18 19:50:29.07 +0800 CST | 2020-02-18 19:50:29.084 +0800 CST | synced | drop database test_ddl_jobs | | 1715 | test_ddl_jobs | | create schema | public | 1714 | 0 | 0 | 2020-02-18 19:45:54.182 +0800 CST | 2020-02-18 19:45:54.185 +0800 CST | synced | create database if not exists test_ddl_jobs | | 1713 | test | level0 | create table | public | 1 | 1712 | 0 | 2020-02-06 20:32:49.557 +0800 CST | 2020-02-06 20:32:49.563 +0800 CST | synced | CREATE TABLE `level0` (`id` int unsigned AUTO_INCREMENT,`value` varchar(255),`level1_id` int unsigned , PRIMARY KEY (`id`)) | | 1711 | test | level2_1 | create table | public | 1 | 1710 | 0 | 2020-02-06 20:32:49.546 +0800 CST | 2020-02-06 20:32:49.552 +0800 CST | synced | CREATE TABLE `level2_1` (`id` int unsigned AUTO_INCREMENT,`level3_id` int unsigned , PRIMARY KEY (`id`)) | | 1709 | test | level1 | create table | public | 1 | 1708 | 0 | 2020-02-06 20:32:49.536 +0800 CST | 2020-02-06 20:32:49.541 +0800 CST | synced | CREATE TABLE `level1` (`id` int unsigned AUTO_INCREMENT,`value` varchar(255),`level2_id` int unsigned,`level2_1_id` int unsigned , PRIMARY KEY (`id`)) | | 1707 | test | level2 | create table | public | 1 | 1706 | 0 | 2020-02-06 20:32:49.527 +0800 CST | 2020-02-06 20:32:49.532 +0800 CST | synced | CREATE TABLE `level2` (`id` int unsigned AUTO_INCREMENT,`level3_id` int unsigned , PRIMARY KEY (`id`)) | | 1705 | test | level3 | create table | public | 1 | 1704 | 0 | 2020-02-06 20:32:49.518 +0800 CST | 2020-02-06 20:32:49.523 +0800 CST | synced | CREATE TABLE `level3` (`id` int unsigned AUTO_INCREMENT,`name` varchar(255) , PRIMARY KEY (`id`)) | | 1703 | test | level0 | drop table | none | 1 | 1554 | 0 | 2020-02-06 20:32:49.5 +0800 CST | 2020-02-06 20:32:49.513 +0800 CST | synced | DROP TABLE `level0` | | 1702 | test | level1 | drop table | none | 1 | 1697 | 0 | 2020-02-06 20:32:49.479 +0800 CST | 2020-02-06 20:32:49.493 +0800 CST | synced | DROP TABLE `level1` | +--------+---------------+------------+---------------+--------------+-----------+----------+-----------+-----------------------------------+-----------------------------------+--------+--------------------------------------------------------------------------------------------------------------------------------------------------------+ 10 rows in set (0.00 sec) ``` Group by: ``` mysql> select db_name,count(*) from DDL_JOBS group by db_name; +---------------+----------+ | db_name | count(*) | +---------------+----------+ | test_ddl_jobs | 3 | | test | 1119 | | mysql | 23 | +---------------+----------+ 3 rows in set (0.07 sec) ``` ### Check List Tests - Unit test - Integration test Code changes - Has exported variable/fields change