Behaviour of OLA-HD and ocrd_olahd_client regarding previous works.

[joschrew]

Currently a previous work must be specified via it's PID. So to update a work the latests PID of the work has to be known. Then the PID must be provided when uploading an update of an existing OCRD-ZIP.
We should think about using the ocrd_identifier to reference a previous work. Olahd could store the ocrd_identifier of each work in its database and ensure it's uniqueness. Then when uploading a work, the ocrd_identifier of a previous work (instead of the PID) could be specified to make it clear that this is an update of an existing OCRD-ZIP.
Benefit: Update handling becomes easier because only the ocrd_identifier has to be known and storing multiple versions can be automated. Disadvantage: an ocrd_identifier must be unique and already used ocrd_identifiers cannot be reused.