This is a prerequisite for T637.

For some changes (like Create, Modify, Move, ...) the server should send not only the metadata describing the change but the actual changed Entity as part of the message. This would save us one roundtrip to the server to request the Entity. Even more, it would allow the server to optimize and only retrieve the Entity once from the database and send it to all subscribers, unlike now when all subscribers ask for the Entity and the server needs to fetch and send it to each subscriber independently. This should give us nice speed up especially during sync.