[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?
well if other event listeners do any sort of modification, they would be triggered again. if they all only note that the event took place, this flush should not do anything.