The cron scheduler has been made more robust against changes in the
system clock (caused by e.g. DST changes, NTP syncs or manual setting).
A more granular management for cronjob events has been introduced, now
supporting a `TIME_SYNC` event besides the usual `STOP`. When the cron
scheduler detects a system clock drift (i.e. the timestamp offset before
and after a blocking wait is >1 sec) then all the cronjobs are notified
and forced to refresh their state.
