Support iteration of data using both name and column index.

As written here: https://github.com/mysql-d/mysql-native/pull/188#issuecomment-448777316

It was suggested to make an issue of its own about this, which I agree with.

So basically it would be a nice addition to iterate through the column names without multiple calls to retrieve the names.

This will help to simplify implementation, but should also increase performance.