This patch implements a very basic scheduler for the ACME tasks.
The scheduler is a task which is started from the postparser function
when at least one acme section was configured.
The scheduler will loop over the certificates in the ckchs_tree, and for
each certificate will start an ACME task if the notAfter date is past
curtime + (notAfter - notBefore) / 12, or 7 days if notBefore is not
available.
Once the lookup over all certificates is terminated, the task will sleep
and will wakeup after 12 hours.
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