dnsdist: Use a separate lock for accessing the pool's servers

[rgacogne]

Short description

We used to hold the Lua lock while applying the load-balancing policy to select a backend, which is only needed by Lua policies, not core ones. We do need a lock to make sure that the vector of servers is not altered under our feet, but a per-pool read-write lock is enough and reduces contention a lot, especially when the maintenance thread is doing some heavy-lifting.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled and tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[rgacogne]

This PR supersedes #6190.