dnsdist: Fix a crash when a DoH responses map is updated at runtime

[rgacogne]

Short description

We now use reference counting so that the response map is only destroyed after all readers are done with it. Previously updating the map meant that the underlying data was destroyed right away, while it might have been in use by DoH connection handler threads.
Fixes #9927 in my tests.

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)