cache locking allows bypassing herd avoidance

[kylegibson-rldatix]

1. two tasks (or more) enter apply_async at roughly the same time.
2. cached result and herd avoidance checks pass for both
3. both tasks hit the cache lock
4. only 1 task is allowed in at a time, that task is scheduled
5. the remaining tasks are still scheduled because they are after the herd avoidance check

https://github.com/PolicyStat/jobtastic/blob/master/jobtastic/task.py#L199