implement remove cascade support for Referrers/ReferenceMany

supersedes #302, fixes #310 and fixes https://github.com/sonata-project/SonataDoctrinePhpcrAdminBundle/issues/142

* [x] add test for ``Referrer`` remove cascade
* [x] fix remove cascade for ``ReferenceMany``
* [x] add test for ``ReferenceMany`` remove cascade
* [x] fox remove cascade ``ReferenceMany``
* [x] add tests for multiple ``flush()`` calls without a ``clear()``