Scheduled BACs don't start if the BACK worker is busy processing other BACs

[prioux]

The BackgroundActivityWorker's do_regular_work() loop invokes the schedudling preparation method BackgroundActivity.activate_scheduled only once, at its beginning.

This means that if there are any BACs that are active and InProgress, and they have many items to process over a long period of time, any scheduled BAC will not be activated until they are finished.

The code internal 'busy' loop within do_regular_work() should maybe invoke activate_scheduled once per 2 minutes or so, just to make the scheduled BACs enter the processing rotation (they will be picked up by the ActiveRecord relation descvribed in todo_base.

This is an annoyance, and doesn't lead to any data problem except that we're not truly using as much parallelism as we could get. The inner loop is supposed to redistribute its workload uniformly among all active BACs.

[prioux]

I committed a fix for this today: 4b6687d