`ns` records aren't replicated to secondary

As they're [considered](https://github.com/RyanGibb/eon/blob/5a56fd3173a3f123d99cb674cb28c133e0cfc263/lib/cap/primary.ml#L136) 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.
```