`ns` records aren't replicated to secondary

[RyanGibb]

As they're considered glue records and not part of the zone.

E.g.

dig ns freumh.org @freumh.org +short
ns1.freumh.org.
ns1.sirref.org.
ryan@gecko:~ %
dig ns freumh.org @sirref.org +short
ns1.sirref.org.