UPnP is now used to request a port on the NAT be forwarded to the local machine.
This obviously only works for NATs that support UPnP, and thus is not a panacea
for all NAT-related connectivity issues.
Notable hacks in this patch include a transmit/receive UDP socket pair to work
around current protocol limitations whereby the full node assumes its peer can
receive on the same UDP port it transmitted from.
Rob Walker
Michael Vines
Greg Fitzgerald
Tyera Eulberg