support forwarding to multiple targets

This is a proof of concept of forwarding different domains to different recursive DNS providers to increase privacy / reduce centralization (from POV of the forwarding resolver).

From a quick test, 8.8.8.8 doesn't seem to like it for some reason and occasionally doesn't respond (observed with both 53/udp and 853/tls). Perhaps there's something weird with my network, but more testing is definitely required. (unrelated, TLS_FORWARD to 8.8.8.8 doesn't work due to Google-imposed rate limiting)