Add an after_all_successful_events callback

[alecvn]

It seems to me a callback is required which one can assign to all events and only triggers if the event was actually successful. The after_all_events callback is fairly lacking in that, as far as I can tell, there is no way of knowing whether the events were successful or not.

An after_all_successful_events callback, which is called after all the individual events' success callbacks would be ideal for this scenario.

If one then needs to wait for the data to actually be persisted to the database one can use something like AfterCommitEverywhere.