I'm open to changing the name, but I think `no_job_available` is better, for a few reasons:

* `wait_to_pop_next_job` suggests it waits until the next job is available.  It doesn't.  It just sleeps for a bit.
* My intention here was to make this a callback -- i.e. it defines what to do when no job is available.  `wait_to_pop_next_job` is a good name for what the base implementation of the callback does, but is specific to the base implementation.  Consider that elsewhere we override this to have it call `shutdown` and not wait at all. That makes sense with a name like `no_job_available` (i.e. "when no job is available, shut down") but would be quite odd for a method named `wait_to_pop_next_job`.