dnsdist: aligned atomics

[rgacogne]

Short description

Rebased version of #9455. I also squashed the commits a bit so that every commit compiles.

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)

[omoerbeek]

Since this is originating from myself I'm hardly the person to approve. But it looks good :-)