As it happens, this is a pretty major change, because I misunderstood
how the protocol actually works. Rather than having a single core
command channel and then a series of offshoots, SOCKSv5 does a separate
handshake for each individual command, and then uses the command stream
as a data stream. So ... whoops. So now the `SOCKSv5Server` sits on a
listener, instead, and farms each of the connections out to a task.
