MEDIUM: sched: do not run a same task multiple times in series

MEDIUM: sched: do not run a same task multiple times in series

There's always a risk that some tasks run multiple times if they wake
each other up. Now we include the loop counter in the task struct and
stop processing the queue it's in when meeting a task that has already
run. We only pick 16 bits since that's only what remains free in the
task common part, so from time to time (once every 65536) it will be
possible to wrongly match a task as having already run and stop evaluating
its queue, but it's rare enough that we don't care, because this will
be OK on the next iteration.