PIM-3365: allow the product saver to bulk products

TODO: 
 - [ ] EE PR

| Q                    | A
| -------------------- | ---
| Bug fix?             | N
| New feature?         | N
| BC breaks?           | Y
| CI currently passes? | Y
| Tests pass?          | Y
| Scenarios pass?      | running
| Checkstyle issues?*  | N
| PMD issues?**        | N
| Changelog updated?   | Y
| Fixed tickets        | -
| DB schema updated?   | N
| Migration script?    | -
| Doc PR               | -
$objectManager ?