BibHarvest: add priority to uploads
BibHarvest: add priority to uploads

* Adds a high priority (6) to BibUpload tasks spawned from
  OAI harvesting post-processes, to ensure more immediate execution
  of the harvesting tasks.

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