dnsdist: Lowercase the name blocked by a SMT dynamic block

[rgacogne]

Short description

This does not change the existing behavior since we are doing a case-insensitive comparison but it's nicer when generating metrics about the dynamic block rules.

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)