[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.
I'm not familiar with the UoW and so, the identity map but removing the proxy entity from the entity map does not result to a no deletion of the entity? or at least, the no triger of the postRemove event for the identity map removed?