Currently a TcpPing using the target address as source is used to
determine wheter or not an IP address belongs to a node. This does not
work for IPv6 anymore (I'm not sure why) as binding to IPv6 localhost as
source for the TCP connection in TcpPing still results in a successful
connection, even in case the target address is non-local.

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