dnsdist: Mark the remote member of DownstreamState as const

[rgacogne]

Short description

While investigating #6664, I noticed that we pass the remote member to functions that could alter it, like Socket::recvFrom(). At the moment I don't see how it could have led to having a different port in the end, since we use a connected socket, but it's still not very clean.
As there is no reason to alter remote and some other members after the object construction, this PR marks them as const so we are prevented from passing them to a function that might alter their content.

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)