TiDB should use separated Allocators for the auto_increment column value and the row ID

execute SQL statements as follows:

```
create table t (c int auto_increment, key(c));
insert into t values();
insert into t values();
select * from t;
```

tidb result:

```
+---+
| c |
+---+
| 1 |
| 3 |
+---+
```

mysql result:
```
+---+
| c |
+---+
| 1 |
| 2 |
+---+
```
