BibUpload: duplicate subfields in bibrec_bibxxx

* bibrec_bibxxx tables are not designed to support
  duplicate subfields within the same field of a record.
  However this was not enforced in the table constraints.
  Amends bibuplod to not generate duplicate rows in
  bibrec_bibxxx tables and corrects these table constraints.
  (closes #1714)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
The duplicate field situation should be rather rare, so doing things *in situ* is preferable in my eyes.  

(And if something goes terribly wrong during the upgrade recipe, like a power cut or something, people can resort to backups.)