bgp: keep and use nexthop value as a string internally

Instead of BGPPathAttributeNextHop, keep nexthop value as a string.
Convert it to string as we get Update msg.

Signed-off-by: ISHIDA Wataru <ishida.wataru@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
packet/bgp: add route distinguisher classes and refine codes

Signed-off-by: ISHIDA Wataru <ishida.wataru@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
bgp: fix bug of exchanging paths between vrf and global_table

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

Signed-off-by: ISHIDA Wataru <ishida.wataru@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
bgp: remove unnecessary OutgoingRoute attribute 'bgp4_format'

Attribute 'bgp4_format' was intended to be used to decide whether to
construct UPDATE msg in bgp4 format or mpbgp format.
But we can decide this simply by checking the path type which we are going
update. If the path type is IPv4, we use bgp4 format. In other cases,
we use mpbgp format.

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