move results db queries into a dedicated method

Pretty easy one here. This makes it easier to adapt Searchkick to other database drivers. If someone has a database with unique query syntax, they shouldn't have to override the entire `results` method. In my case, it lets me do this:

```ruby
    def results_query(records, grouped_hits)
      if records.respond_to?(:from_class)
        records.array_load(grouped_hits.map { |hit| hit['_id'].to_i })
      else
        super
      end
    end
```