dnsdist: Do not create socket/thread for fake DS in client mode

dnsdist: Do not create socket/thread for fake DS in client mode

While parsing the configuration in client mode, we create a fake
DownstreamState for each newServer() call, because we need it to
return a valid DownstreamState object. Unfortunately this leads
to the creation of a socket for 0.0.0.0, and a subsequent
connection attempt.
We now detect that the address does not make sense in this context
and do not create the associated socket.
Closes #3257.