Refactor service check scheduling

Always reschedule a service check one check_interval in the future
directly at event handler execution. If we want something closer,
replace the event, so we can guarantee that one service check event is
scheduled at all time. If we can do that, we know for sure that we
don't forget about service checks.

Signed-off-by: Max Sikstrom <msikstrom@op5.com>