* Set SOA record TTL as min of TTL/MINIMUM
According to
https://tools.ietf.org/html/rfc2308#section-3:
The TTL of this record is set from the minimum
of the MINIMUM field of the SOA record and the
TTL of the SOA itself, and indicates how long a
resolver may cache the negative answer.
Set the TTL accordingly so as to not always
negative cache SOA records for 300 seconds.
* Inline min func
* Update SOA record tests
* Fix types