add functions to 550-bind_dnssec.inc.php (see https://www.howtoforge.com/community/threads/dnssec-problem-expire-too-soon-ispconfig-3-1.74682/#post-351381)

DNSSEC-Cronjob should only run on ispconfig master as we are working with datalog and not accessing things directly

improved crojob

Fixed cron to update serial and not sign directly. This now issues a job for the cron server which will then generate the zone and sign it.
cleaner solution AND fixes a possible replication (zone-transfer) issue.