[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?
event listeners should also take care to not go into endless loops. but this should work so if you can investigate why it does not, that would probably fix some other issues. btw, is there not some public method to ask the dm / uow if there is anything to flush?