Key double-checking for ODoH

[jedisct1]

https://www.ietf.org/archive/id/draft-schwartz-ohai-consistency-doublecheck-03.html suggests connecting twice to the relay in order to retrieve the keys: once to get the (possibly) cached content, and using the relay as a TCP proxy to connect to the upstream server.

This forces ODoH relays to also support acting as TCP relays. Something that makes me feel a little bit anxious. And an ODoH relay that would like to send different keys to different targets can pretend not to support TCP relaying.

Still something we may want to implement, and make optional.