revisionid should be generated on client, not metacat
Currently metacat assigns a revisionid when allowing updates of metadata
documents. This should be changed so that the client specifies the right
revisionid. If a client specifies a revision that is less than the revision
stored on metacat, the operation must fail with an appropriate error message
(the client can then retrieve the recent revision and resubmit). This requires
parallel changes in Morpho.