Builder should error when a required field is defined after an optional field

For example:

```
dbNamespace.register({
    name: 'entry',
    fields: [
      { name: 'id', type: 'string', required: true },
      { name: 'metadata', type: 'string', required: false },
      { name: 'createdAt', type: 'date', required: true }
})
```

is not be allowed. So better to error while building.