PLS: schedule()

First check the realtime priority list
- If non-empty select a realtime task
Determine range of lists to scan
- Always start at the top (highest priority)
- Choose the default next task to run
  - If current is still runnable, it becomes the default task and the bottom range of lists is set to that of current (no task on a lower priority list can receive enough affinity boost to overcome current)

First check the realtime priority list