spooler exponential retry delay
Currently, djcall offers a max_attempts feature in djcall/models.py.
We need to be able to configure an exponential delay.
To close this feature, contribute exponential delay to djcall:
- add "retry seconds" field
- add "exponential factor" field
Here's another app that does it: https://github.com/Bahus/uwsgi_tasks/blob/00954198c4d6993960f40b02a388d27978f3f798/uwsgi_tasks/tasks.py#L545-L547