Use `as of` clause to query `information_schema` history

Right now there's no way to use SQL to get foreign key/indexes for a revision in a database. It has a `table_schema` column but it only shows the revision database that's currently being used
```sql
mysql> use testtest;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> select table_schema, table_name, index_type from information_schema.statistics where table_name="t1";
+--------------+------------+------------+
| TABLE_SCHEMA | TABLE_NAME | INDEX_TYPE |
+--------------+------------+------------+
| testtest | t1 | BTREE |
+--------------+------------+------------+
1 row in set (0.04 sec)

mysql> use `testtest/main`;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> select table_schema, table_name, index_type from information_schema.statistics where table_name="t1";
+---------------+------------+------------+
| TABLE_SCHEMA | TABLE_NAME | INDEX_TYPE |
+---------------+------------+------------+
| testtest | t1 | BTREE |
| testtest/main | t1 | BTREE |
+---------------+------------+------------+
2 rows in set (0.04 sec)

mysql> use testtest;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> select table_schema, table_name, index_type from information_schema.statistics where table_name="t1" and table_schema="testtest/main";
Empty set (0.04 sec)
```

Would be helpful to use an AS OF clause to query `information_schema` at different database revisions, like:
```
select * from information_schema.statistics as of "testtest/main" where table_name="t1" ;
```

This would be useful for the hosted workbench, where we display foreign key and index information for each branch