Fixes memory leak on generic Doctrine Writer

The writer uses now the ProductCacheClearer
Merge branch '1.2' into fix/PIM-3121

Conflicts:
	CHANGELOG.md
Merge branch 'master' into fix/PIM-3121

Conflicts:
	CHANGELOG.md
Merge pull request #1433 from akeneo/fix/PIM-3121

Fixes memory leak on generic Doctrine Writer