[BC Break] Fix doctrine iterators for sf2.2 & 2.3 compatibilty

Replaces the deprecated `Symfony\Component\Form\Util\PropertyPath` using
`Symfony\Component\PropertyAccess\PropertyAccess` new method of
accessing properties, making it compatible with sf2.3 (and thus 2.2)

Note that the `PropertyAccess::getPropertyAccessor();` will be deprecated in sf3.0 in favor of `createPropertyAccessor()` but it is kept in order to maintain the 2.2 compatibility.