unified options for encoding and decoding

protocols/bgp/packet/decoder.go

protocols/bgp/packet/decoder_test.go

protocols/bgp/packet/encoder.go

protocols/bgp/packet/options.go

protocols/bgp/packet/path_attributes.go

protocols/bgp/packet/path_attributes_test.go

protocols/bgp/server/fsm.go

protocols/bgp/server/fsm_established.go

protocols/bgp/server/fsm_open_confirm.go

protocols/bgp/server/fsm_open_sent.go

protocols/bgp/server/update_helper.go

protocols/bgp/server/update_sender.go

protocols/bgp/server/update_sender_add_path.go

protocols/bgp/server/withdraw.go