EventType versioning

EventTypes need some versioning. Proposed: SemVer - breaking major, non-breaking minor versions

Read models should be able to use event types with same major version without changes (but then they don't use the additions).

Event Types should never need to be 'migrated'. Once an event is stored, it never changes.

• this means that every event also needs to store it's own version, possibly as an URI to the description of that event type