dnsdist: Prevent a copy of a pool's backends when selecting a server

[rgacogne]

Short description

It makes a noticeable difference as soon as the number of backends in a pool increases to more than 8 backends, and a huge difference when there is a large number of backends.

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]

This reads good (I checked the const usage in particular) and fits what we have seen.

[omoerbeek]

The regress failures a bit worrying, though

[rgacogne]

The regress failures a bit worrying, though

Both failures seemed unrelated and the tests passed after a restart.