dnsdist: Check response validity over TCP, more cache fixes

dnsdist: Check response validity over TCP, more cache fixes

- Add `unsetCache()` to remove the cache from a pool
- Check the response size before caching it, and make no
assumption when getting it from the cache
- Check that the response is larger than sizeof(dnsheader) over
TCP too
- Check that the response matches the query over TCP too, because
we reuse downstream connections