bgp: add route server function to bgpspeaker

implemented draft-ietf-idr-ix-bgp-route-server-05 partially.

you can enable this function like below

>    speaker = BGPSpeaker(as_number=64512, router_id='10.10.0.1')
>    speaker.neighbor_add('10.0.0.1', 64514, is_route_server_client=True)
>    speaker.neighbor_add('10.10.0.2', 64513, is_route_server_client=True)

add multiple rib function is a future work

Signed-off-by: ISHIDA Wataru <ishida.wataru@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>