rec: Don't add non-IN records to the cache

[rgacogne]

Short description

While processing records sent by an authoritative server, some of them could have been added to the cache even though they have a different class than IN. We should simply skip these records.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled and tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)
• checked that this code was merged to master