Merge pull request #125 from bio-routing/feature/refactor_mpunreach_nlri

protocols/bgp/packet/mp_reach_nlri.go

protocols/bgp/packet/mp_unreach_nlri.go

protocols/bgp/packet/mp_unreach_nlri_test.go

protocols/bgp/packet/nlri.go

protocols/bgp/packet/nlri_test.go

protocols/bgp/packet/path_attributes.go

protocols/bgp/packet/path_attributes_test.go

protocols/bgp/packet/update.go

protocols/bgp/server/fsm_address_family.go

protocols/bgp/server/update_sender.go