Add client.socket.domain for symmetry with server.socket.domain

[trask]

Originally from #206 (comment)

[trask]

One idea @lmolkova and I briefly discussed (we're not sure if it's a good idea or not yet) is that since server.socket.domain is very specific to proxy use case, maybe it could be renamed to proxy.address?

[Oberon00]

It could be a proxy or reverse proxy. How about peer.address? (Only half joking)

[trask]

server.socket.domain has been removed now (it was not carried over to the new network.peer/local.*)

the thought is to revisit the use case for server.socket.domain (forward proxies) in the future, potentially with new attribute(s), e.g. proxy.address

so I think this can probably be closed