Rather than having standalone proxies determine their NAT type by
conducting the NAT behaviour checks in RFC 5780, use the remote probe
service instead.
We expect one of these at the end of just about every proxy session, as
the Conns in both directions are closed as soon as the copy loop
finishes in one direction.
Closes#40016.
The client and proxy use the net/http default transport to make round
trip connecitons to the broker. These by default don't time out and can
wait indefinitely for the broker to respond if the broker hangs and
doesn't terminate the connection.
This will allow browser-based proxies that are unable to determine their
NAT type to conservatively label themselves as restricted NATs if they
fail to work with clients that have restricted NATs.
