[#60] cmd/neofs-node: Implement primary network State instance

Define networkState structure and implement netmap.State interface on it.
Set epoch number on NewEpoch notification from chain. Provide
WithNetworkState option to Put service constructor.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

cmd/neofs-node/config.go

@@ -163,6 +163,8 @@ type cfgNetmap struct {
 	parsers map[event.Type]event.Parser
 
 	subscribers map[event.Type][]event.Handler
+
+	state *networkState
 }
 
 type BootstrapType uint32