Cache table schema

See #57

```
make demo
...
Queries performed: 100
```

### Duplicated queries

```bash
DEBUG=1 make demo 2>&1 | grep indexdigest.database.query | sed 's/ INFO /#/g' | awk -F# '{print $2}' | sort | uniq -c | sort -rn
...
      2     SHOW CREATE TABLE 0075_some_guy_test_table
      2     SHOW CREATE TABLE 0074_bag_of_ints
      2     SHOW CREATE TABLE 0034_querycache
```