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>
If you run explain on `SELECT DISTINCT * FROM table_name` you will see that you might miss an index there, that's why it's doing `filesort` which is ... slow on 1M+ of rows ;)