dnsdist: Keep the TCP connection open on cache hit, generated answers

dnsdist: Keep the TCP connection open on cache hit, generated answers

We used to close the TCP connection right away on cases where that did
not make sense:
- on a cache hit
- on a self-generated answer
- on a servfail answer caused by the lack of usable downstream servers

We still close the TCP connections on drops, dynamic blocks, lack of
usable downstream servers without `setServFailWhenNoServer()` set,
invalid queries, network errors..