Transactional event updates

- Updates to event objects are now transactional. Any updates to the “participants” field will be ignored. Instead, in order to update the participants field for an existing event the /event/<key>/participants CRUD must be used.
- User object edits would also be transactional with this merge if we didn’t have the non-transactional update in the OAuthFilter... which can’t be simply changed because of a bootstrapping issue with getCurrentUserKey() for the modification timestamp.