[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?
Have reimplemented this early exit from the ORM. Unfortunately it doesn't seem to avoid the infinite loop in the EventManagerTest. Will have to come back to it.