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>
You know what: that is not slower but terribly slower in the normal case (e.g. one million records). With the previous pythonic implementation only the sparse duplicates were removed while here, as soon as there is a duplicate the whole table is copied... I am not sure it's really worth.