Use nonblocking DNS for clients

[slipher]

cu-kai and freem report having seen O(10s of seconds) hangs due to the client's server list queries. This is presumably caused by DNS. A possible scenario is wanting to join a LAN game but getting stuck at the automatically initiated internet list refresh when opening the server list.

We already have an API for DNS resolution on a separate thread which is used by the server. We could use it for clients too.