The server tries to merge conflicting saves of
the same layer.
What it does:
- use the `If-Unmodified-Since` header to check
if changes happened to the stored data ;
- Compare the incoming version with its reference version
to get a diff.
- Reapply the diff on top of the latest version.
- If the merge is not possible, return a
"422 Conflict" HTTP response.
- If the merge worked, return the merged document,
to be updated by the client.
