[WIP] fix referrers cascade persist

this now almost works, except that we seem to hit some jackalope bug which i seem to confused to understand.

note that atm this breaks two other tests:

* Doctrine\Tests\ODM\PHPCR\Functional\EventManagerTest::testTriggerEvents => wft is https://github.com/doctrine/phpcr-odm/blob/master/tests/Doctrine/Tests/ODM/PHPCR/Functional/EventManagerTest.php#L129 trying to achieve? is this leftover code from when referrers where not cascading yet? if we really want to test something about referrers there, we now need to define the filter option to the mapping as cascading persist only works if we know the name of the referring property.
* Doctrine\Tests\ODM\PHPCR\Functional\MergeTest::testMergeNewDocument => here i am not sure why this fails and did work before