* 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