Support for resolving hostnames to IPv6 addresses

This patch enables IPv6 name resolution by using socket.getaddrinfo
instead of socket.gethostbyname_ex.

It renames the HostInfo class to Hostname and unifies its use throughout
the code. This is achieved by using static calls where no object is
needed and removes some obsolete code.

For now, we just resolve to IPv4 addresses, but this will change once it
is needed.

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