lib: improve late timer warnings

lib: improve late timer warnings

Refactor the "timer getting executed too late" warning:

- warning threshold is now adjustable
- check is performed when event actually executes, rather than when it's
  thrown on ready list
- ignore_late_timer replaced with threshold = 0
- system load averages printed in log message
- warning ratelimited to once per 10s rather than once per poll()

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>