BibSched: bibupload max_priority check

* Optimises check for bibupload task priority normalization, avoiding
  useless SQL UPDATE statements in case normalization is not needed.

Reported-by: Ferran Jorba <Ferran.Jorba@uab.cat>
Co-authored-by: Tibor Simko <tibor.simko@cern.ch>
Merge branch 'maint-1.1'

* maint-1.1:
  BibSched: bibupload max_priority check