Import CSV rows one at a time (#3450)

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.

modules/backend/models/ImportModel.php

@@ -142,7 +142,7 @@ abstract class ImportModel extends Model
         }
 
         $result = [];
-        $contents = $reader->fetchAll();
+        $contents = $reader->fetch();
         foreach ($contents as $row) {
             $result[] = $this->processImportRow($row, $matches);
         }