Should we rename `mysql.tidb.tikv_gc_life_time` to `tidb_gc_life_time`?

## Enhancement

### 1. Minimal reproduce step (Required)

```
mysql> select @@tidb_gc_life_time;
+---------------------+
| @@tidb_gc_life_time |
+---------------------+
| 10m0s |
+---------------------+
1 row in set (0.01 sec)
mysql> set @@tidb_gc_life_time=20m0s;
ERROR 1229 (HY000): Variable 'tidb_gc_life_time' is a GLOBAL variable and should be set with SET GLOBAL
mysql> set global tidb_gc_life_time=20m0s;
Query OK, 0 rows affected (0.04 sec)

mysql> select * from mysql.tidb where variable_name="tikv_gc_life_time";
+-------------------+----------------+----------------------------------------------------------------------------------------+
| VARIABLE_NAME | VARIABLE_VALUE | COMMENT |
+-------------------+----------------+----------------------------------------------------------------------------------------+
| tikv_gc_life_time | 20m0s | All versions within life time will not be collected by GC, at least 10m, in Go format. |
+-------------------+----------------+----------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

```

### 2. What did you expect to see? (Required)

I wish the valiable_name `tikv_gc_life_time` in `mysql.tidb ` and system variable `tidb_gc_life_time` share the same name.
### 3. What did you see instead (Required)
They are different which may mislead me.

### 4. What is your TiDB version? (Required)
master