Add configure-time switch for split queries

And expand cli.GetClient() to allow opening the query socket, instead
of the main master socket.

Finally, enable the query socket use in gnt-cluster version, since
that is already implemented fully in Queryd.hs/hconfd.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>