Always use address instead of hostname in rpc.Client

In light of the upcoming IPv6 support, this patch enables the rpc.Client
to always use a node's address to connect to it. This is necessary as we
do not want to rely on name resolution to connect to the correct IP
address on a dual-stack machine.

Signed-off-by: Manuel Franceschini <livewire@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>