MEDIUM: tasks: clean up the front side of the wait queue in wake_expired_tasks()

MEDIUM: tasks: clean up the front side of the wait queue in wake_expired_tasks()

Due to the way the wait queue works, some tasks might be postponed but not
requeued. However when we exit wake_expired_tasks() on a not-yet-expired
task and leave it in this situation, the next call to next_timer_expiry()
will use this first task's key in the tree as an expiration date, but this
date might be totally off and cause needless wakeups just to reposition it.

This patch makes sure that we leave wake_expired_tasks with a clean state
of frontside tasks and that their tree's key matches their expiration date.
Doing so we can already observe a ~15% reduction of the number of wakeups
when dealing with large numbers of health checks.

The patch looks large because the code was rearranged but the real change
is to take the wakeup/requeue decision on the task's expiration date instead
of the tree node's key, the rest is unchanged.