dnsdist: Speed up cache hits by skipping the LB policy when possible

dnsdist: Speed up cache hits by skipping the LB policy when possible

We use to execute the load-balancing policy to select a backend before
doing the cache lookup, because in some corner cases the selected
backend might have settings that impact our cache lookup. In practice
most configurations have a consistent set of settings for all servers
in a given pool, so it makes no sense to waste CPU cycles selecting a
backend if we are going to get a hit from the cache.
This PR adds a bit of code to check if a pool is in a consistent state,
and if it is it delays the execution of the load-balancing policy to
after the cache lookup, skipping it entirely for cache hits.

Signed-off-by: Remi Gacogne <remi.gacogne@powerdns.com>