dnsdist: Stop over-allocating for DoH queries

[rgacogne]

Short description

It used to be necessary because we had a fixed buffer and might have needed the space for cache hits, but nowadays we can resize later if needed so stop wasting memory.

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)