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

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

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.