Expand the retry functionality

We're using the retry functionality in Commerce License Billing. It's been working well, but it needs two changes:

- We shouldn't retry on every failure, the retry should be requested by returning a special status.
This allows the module that defined the queue to control the retry, and distinguish between soft and hard failures.
- Allow unlimited retries ('max attempts' not defined)
This also makes sense if the control of the retry is now in the hands of the module.