dnsdist: Refactor load-balancing policies

dnsdist: Refactor load-balancing policies

Since we no longer need to increase the reference counter of the
returned backend (the runtime configuration cannot be updated be
updated under our feet anymore), we can return the position of the
selected backend in the initial array instead, significantly
reducing the performance cost of the load-balancing policies.

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