PIM-2345 : clear the object manager during CSV export to avoid memory

| Q                    | A
| -------------------- | ---
| Bug fix?             | Y
| New feature?         | N
| BC breaks?           | Y
| CI currently passes? | Y
| Tests pass?          | ?
| Scenarios pass?      | ?
| Checkstyle issues?*  | N
| PMD issues?**        | N
| Changelog updated?   | Y
| Fixed tickets        | PIM-3245
| DB schema updated?   |
| Migration script?    |
| Doc PR               |