With this patch we stop scheduling when we should write statistics using a
single timestamp in run_scheduled_events(). Instead, we remember when a
statistics interval starts separately for each statistic type in geoip.c
and rephist.c. Every time run_scheduled_events() tries to write stats to
disk, it learns when it should schedule the next such attempt.
This patch also enables all statistics to be stopped and restarted at a
later time.
This patch comes with a few refactorings, some of which were not easily
doable without the patch.
