NEW: roll_srs()- apply new SRS secret to thwart replay attacks. This may be automated by setting [mail] => srs_autoroll. See SECURITY.md (email)
FIX: version filter (Nextcloud)
CHG: schedule() takes null value to disable scheduling, cleans up nesting within _cron() methods (Cronus)
CHG: empty cron buffer on each module iteration (apnscpd)
CHG: separate _cron() hook into "Tasking" interface