Separate processings in multiple queues

[rafamanzo]

I suggest three queues:

• first_processing: gives higher priority for new repositories
• periodic: isolate periodic processings making easier to prevent duplications
• reprocess: allows low priorities for such tasks

Would be nice that the foreman export task could generate one worker for each queue (and we could add more workers to priority queues).

We would probably work on this by creating ProcessingJob inheritances properly.