Group events by aggregate version

[dlidstrom]

When committing events to an aggregate, group them by the aggregate version. This allows us to see which events where committed together and will help when considering concurrency and merging of events. A simple version property on the event will do, I think.

[dlidstrom]

I'm leaving this for now. In order to handle concurrency it seem the event store needs to handle its own document sessions. That would complicate things for the user and I am not sure the cost in terms of complexity is worth it. If you have an idea, please let me know!