Prevented removed provider events to be ignored

I believe that current implementation prevents removed provider events to be dispatched because it relies on unit of work's identity map which is truncated when entity is removed (see https://github.com/doctrine/doctrine2/blob/master/lib/Doctrine/ORM/UnitOfWork.php#L1307).

This PR suggests, as beberlei does in gist.github.com/beberlei/53cd6580d87b1f5cd9ca, to store persist, updated and removed entities in the listener and to rely on that.