BibSched: fix waiting bibupload task priorities
BibSched: fix waiting bibupload task priorities

* BibUpload tasks are always executed in order. When a bibupload with high
  priority is submitted after a bibupload with low priority the former was
  subject to execution (based on priority) of the latter, the priority
  of the former was kind of ignored.  This fixes the behaviour by raising
  all the waiting bibupload priorities to the maximum priority among the
  waiting bibupload tasks.

tiborsimko/invenio,inveniosoftware/invenio,inveniosoftware/invenio,tiborsimko/invenio