[DDC-3179] EntityNotFoundException on the postRemove event if the entity is a proxy

This PR tries to fix [DDC-3179](http://www.doctrine-project.org/jira/browse/DDC-3179).

I'm not sure if it is the way to go as this PR has a big drawback: it **always** loads proxies which needs to be removed regardless if there is an postRemove event for it. Anyway, there is no way to determine if there is an event for a specific entity in a lifecycle callback context.

For now, to fix it in our side, we have registered a listener on the preRemove event and load the proxy there.
@guilhermeblanco This PR does not really fix an issue about cascades as they are well handled before this PR. The proxy was only loaded if there is at least one association which needs to be cascaded on the entity.

The issue is if you register a postRemove event on a entity which is a proxy (not initialized and without cascades), then the proxy id is reset to null and then, when you try to access there properties on the postRemove event, the proxy loading fails because there is no more record in the DB and so, make the trigger of the NotFoundEntityException.

Anyway, as I and you pointed out, it is a drastic approach as it loads all proxies which will trigger extra requests (not really required for most use cases)... For now, I have not found an other approach in order to fix it in the ORM itself. So, documenting it as a limitation seems  probably the best solution.