[OPTIM] displace tasks in the wait queue only if absolutely needed

[OPTIM] displace tasks in the wait queue only if absolutely needed

We don't need to remove then add tasks in the wait queue every time we
update a timeout. We only need to do that when the new timeout is earlier
than previous one. We can rely on wake_expired_tasks() to perform the
proper checks and bounce the misplaced tasks in the rare case where this
happens. The motivation behind this is that we very rarely hit timeouts,
so we save a lot of CPU cycles by moving the tasks very rarely. This now
means we can also find tasks with expiration date set to eternity in the
queue, and that is not a problem.