Respond with current revision (as header) of Ditto managed entities on each API call

[thjaeckle]

Currently, the current revision is only returned when e.g. modifying a complete thing (as part of the etag header).

It would however be beneficial if the current revision is always returned as Ditto defined header.

• for modifying requests
• but also for retrieving requests

That way, an implementation can e.g. use the "revision" to do a conditional update in the next request.

[thjaeckle]

It would be also good if we could use _revision in a condition to update things ..
That way, just by using the header and a eq(_revision,42) with the next update, optimistic locking can be achieved. Ditto will reject the update if another change was done in the meantime.