Now that the keymgr tracks the maximum TTL used in a zone, we no longer
need an artifical value when calculating key rollover timings. The
max-zone-ttl value can be set to 0 (unlimited) by default.
This means that the built-in "default" and "insecure" can now be used
on zones that have TTL values higher than 1 day (and no longer fail
to load the zone).
You can still set the max-zone-ttl value to ensure that the loaded
zone does not have a too high TTL (causing the zone fail to load, and
to limit the TTL on incoming updates).
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