dnsdist: Slightly reduce contention around a pool's servers

[rgacogne]

Short description

We only need to take the lock to get the shared pointer, as the actual content is guaranteed not to change, so we do not need to hold the lock while we iterate over the servers list to check whether they are up, or what their current outstanding count is.

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]

You might consider start using std::shared_ptr<const ServerPolicy::NumberedServerVector> From a quick look, this will need a few changes here and there, but not too much. The big benefit is it will make clear that the vector does not change. It might even prevent mistakes...
But OK anyway.

[rgacogne]

Good idea, thanks, I'll give it a try!

[rgacogne]

Done!