Update scan queue

The scan queue has several issues:
- `process_scan` crashes when it loses connection with scancode.io. We should ensure that we handle this exception correctly and stop the queue if we can't communicate with scancode.io
- The code that deals with calling scancode.io should also be updated along the lines of https://github.com/nexB/scancode.io/blob/main/scanpipe/pipes/purldb.py .
- `process_scan` should rotate between the three in-progress scans to see which is available first to be indexed, rather than waiting on one.
- Submitted scans would not always update their status properly and would be stuck in the `scan submitted` stage.