Unifying update sender. Implementing update grouping plus update buffering.

protocols/bgp/packet/bgp.go

protocols/bgp/packet/encoder.go

protocols/bgp/packet/nlri.go

protocols/bgp/packet/options.go

protocols/bgp/packet/update.go

protocols/bgp/server/fsm.go

protocols/bgp/server/fsm_established.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

route/bgp_path.go