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Add SSL functionality

See merge request !2

If the server was signaled to shutdown when the Node was still waiting
to read its configuration, the configReady channel would never fire,
causing a lockup in NewNode().

We can't do low-level splice() since we don't have access to the raw
socket in the TLS case. This should work for clients though (maybe not
for sources).

This allows usage of HTTPS, IPs won't work well in that scenario.

Small costly mistake.

Remove a lot of boilerplate by using the errgroup package instead of
our own custom cumbersome solutions. Control server lifetime with an
outer Context.

It is best to just make the DNS server listen on all non-loopback IP
addresses from all interfaces. The loopback exclusion is just to
integrate nicely with an eventual DNS cache running on the same host.

v2.0

See merge request !1

Causes a slight reduction in boilerplate, and gets rid of a bunch of
custom logic for command-line parsing that is no longer needed.