Strange count query #553 fix

If Grid's query doesn't use ``Doctrine\ORM\Query::HINT_CUSTOM_OUTPUT_WALKER``, query will be wrapped with 

    SELECT count(distinct id) FROM (<original query>)

If query using this hint, ``Doctrine\ORM\Tools\Pagination\CountWalker`` will be added (instead of ``APY\DataGridBundle\Grid\Helper\ORMCountWalker``), like Doctrine's paginator do this. Unfortunatelly, in this case if main query using group by expression, this bug will raises again.