Do not add glues from different parent in delegdb

Do not add glues from different parent in delegdb

When processing a referral, the `cache_delegns()` function was accepting
glues from a different parent. For instance:

```
AUTHORITY
test.example.		NS	ns.test.example.
test.example.		NS	ns.foo.example.
test.example.		NS	ns.bar.

ADDITIONAL
ns.bar.			A	1.2.3.4
ns.foo.example.		A	5.6.7.8
ns.test.example.	A	9.8.7.6
```

In such situation, only the glues for `ns.foo.example.` and
`ns.test.example.` should be used, and the glue from `ns.bar.` should be
ignored as this is not either a sub-domain or a sibling domain, the
parent is different (`bar.` instead of `example.`). This is now fixed.

Sibling glue and cyclic sibling glues are defined in RFC 9471 section
2.2 and section 2.3.