This is the "server-side" part of the new friend finding system,
allowing DHT nodes to store small amounts of data and permit searching
for it. A forwarding (proxying) mechanism allows this to be used by TCP
clients, and deals with non-transitivity in the network.
As a side-effect, DHT now always accepts LAN discovery packets, even
when LAN discovery is disabled. When LAN discovery is disabled, those
packets are ignored.