Fix ProxyQuery error on relational identifiers

Actually, the ProxyQuery can't handle identifier with relational link properly. This PR patch this.

It's a corrected version of #226 and concern issus #225, https://github.com/sonata-project/SonataAdminBundle/issues/1392 and https://github.com/sonata-project/SonataAdminBundle/issues/1000 as far I know.

I think it's quite critical, please have a look! :+1: 

Thanks.

I give you some steps to reproduce the error with the current master version.

If we have an entity with relational identifiers like this:

```php
/**
 * UserBrowser
 *
 * @ORM\Table(name="user_browser")
 * @ORM\Entity
 */
class UserBrowser
{
    const UPDATE_GAP    = 1800;
    const NOTIFY_GAP    = 60;

    /**
     * @var User
     *
     * @ORM\Id
     * @ORM\ManyToOne(targetEntity="Nexy\UserBundle\Entity\User", inversedBy="browsers")
     * @ORM\JoinColumn(name="user_id", referencedColumnName="id")
     */
    private $user;

    /**
     * @var User
     *
     * @ORM\Id
     * @ORM\ManyToOne(targetEntity="AppBundle\Entity\Browser")
     * @ORM\JoinColumn(name="browser_id", referencedColumnName="id")
     */
    private $browser;

    // [...]
```

And a configured admin like this:

```php
/**
 * Class UserBrowserAdmin
 */
class UserBrowserAdmin extends Admin
{
    protected $baseRouteName = 'admin_user_browser';
    protected $baseRoutePattern = 'user/browser';
    protected $datagridValues = array(
        '_sort_by'      => 'lastUsedAt',
        '_sort_order'   => 'DESC',
    );

    protected function configureListFields(ListMapper $list)
    {
        $list
            ->addIdentifier('user')
            ->addIdentifier('browser')
            ->add('lastUsedAt')
            ->add('_action', 'actions', array(
                'actions'   => array(
                    'show'      => array(),
                )
            ))
        ;
    }

    protected function configureShowFields(ShowMapper $filter)
    {
        $filter
            ->add('user')
            ->add('browser.id')
            ->add('lastUsedAt')
            ->add('browser.details')
        ;
    }
}
```

We got this error:

```
An exception has been thrown during the rendering of a template ("[Semantical Error] line 0, col 18 near 'user FROM AppBundle\Entity\UserBrowser': Error: Invalid PathExpression. Must be a StateFieldPathExpression.") in SonataAdminBundle:CRUD:base_list.html.twig at line 33.
```