This reverts commit 24e19af6.

Side effects have been observed since the su_timer_t uses
the monotonic clock to schedule the tasks. Especially,
the function su_timer_set_at() was broken because the user
of this function was using wall clock timestamps whereas the
implementation was working with the monotonic clock. It may be
noted that su_timer_set_at() specification cannot be changed
as it is non-sense to specified a date by a monotonic timestamp.