Shared DHT/UTP/UDP-tracker socket dispatching in Transmission is at
https://trac.transmissionbt.com/browser/trunk/libtransmission/tr-udp.c
event_callback(). Currently I don't do this because github.com/h2so5/utp does
not support UTP sockets backed by a socket out of its control. Also I only
make client requests for UDP-trackers, so no shared socket is required there
unless I want to imply the client port.