MINOR: debug: add "debug dev sched" to stress the scheduler.

MINOR: debug: add "debug dev sched" to stress the scheduler.

This command supports starting a bunch of tasks or tasklets, either on the
current thread (mask=0), all (default), or any set, either single-threaded
or multi-threaded, and possibly auto-scheduled.

These tasks/tasklets will randomly pick another one to wake it up. The
tasks only do it 50% of the time while tasklets always wake two tasks up,
in order to achieve roughly 50% load (since the target might already be
woken up).