[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?
is this change needed? seems like $id is not used afterwards. and we prefer to do assignments outside method calls.