dnsdist: Fix building with boost < 1.56

[rgacogne]

Short description

boost::optional::value_or() has been introduced in 1.56 and we only require 1.53, so stop using it.

Hopefully fixes the master part of #12142.

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)

[Habbie]

I see your portable construct, and I like it, but I also note we use get_value_or in other places. However, that one has been deprecated in Boost, so we shouldn't switch to that.

#12183 backports the part of this PR that applies, and adds another fix in a place that master apparently does not have any more.

[Habbie]

Hopefully fixes the master part of #12142.

There are at least two other problems on master for amazon-2 - one being the specfile now pulling in Boost 1.69 which doesn't work there, and one being the noncopyable include that somebody else also noticed. I hope to fix those two things soon.

[rgacogne]

I see your portable construct, and I like it, but I also note we use get_value_or in other places. However, that one has been deprecated in Boost, so we shouldn't switch to that.

I went for the minimal fix in this PR but I agree we should also stop using get_value_or as it might get removed one day or another. I'll open an issue for that.