dnsdist: Get rid of allocations in the packet cache's fast path

[rgacogne]

Short description

I stumbled upon unneeded memory allocations and copies while doing benchmarks to asses the impact of #8993.
Getting rid of them delivers a nice speed-up, up to ~40% in unrealistic micro-benchmarks, likely much less in practice.

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)