Rec: remember non resolving ns

[omoerbeek]

Short description

If a nameserver name came without glue and also fails to resolve, it cannot be used for anything useful. Remember that fact and use it to throttle (not use) that nameserver for a while.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)