dnsdist: Only record one hit or miss per query in the cache metrics

dnsdist: Only record one hit or miss per query in the cache metrics

The scope-zero feature and the DoH paths can actually do more than
one lookup per query, and until now this led to an increase of the
per-cache metric for every lookup, while the global `cache-hits`
and `cache-misses` metrics were only updated once per query.
This has led to several questions and misunderstandings, so we now
only update the per-cache metrics once per query as well.