Credit @mcharytoniuk.
While importing over 250k records ImportModel kept running into various problems. One of them was too big memory usage - `ImportModel` loaded the complete file upfront (`$reader->fetchAll()`). Simple one-line change to `$reader->fetch()` makes `ImportModel` import CSV file row-by-row and returning an iterator which limits memory usage and allows data to be imported. This change optimizes memory usage and allows much simpler importing of larger files.