allow multiple families per peer

config/peer.go

main_ipv4.go

main_ipv6.go

protocols/bgp/server/family_routing.go

protocols/bgp/server/fsm.go

protocols/bgp/server/fsm_established.go

protocols/bgp/server/fsm_test.go

protocols/bgp/server/peer.go

protocols/bgp/server/server.go

protocols/bgp/server/server_test.go

protocols/bgp/server/update_sender_test.go