bgp: support next_hop_self

Support next_hop_self.
BGPSpeaker can replace a path's next_hop address with its own address
when it sends the path to iBGP peer.

Signed-off-by: Hiroshi Yokoi <yokoi.hiroshi@po.ntts.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
oxm_fields: Add a comment

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
ofproto_v1_3_parser: Handle experimenter OXMs in TableFeatures

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
ofproto_v1_4_parser: Handle experimenter OXMs in TableFeatures

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>