[POC] Added "endFlush" event

Just testing the water -- should we have an end flush event?

I am using this event with success in RoutingAutoBundle to persist additional documents (the redirect routes) but this is a workaround for the fact that the order of persistance is wrong (and refactoring the entire UOW is a big task).

Are there any genuine use cases for this event?
In accordance with what I said earlier, I don't think we should be calling `flush()` if we are modifying the PHCPR session directly. The ODM has no knowledge of the inner state of the PHPCR session.