[#312] netmap: Support NetmapService.NetmapSnapshot RPC

Extend functionality of `NetMap` type. Add `NetMapSnapshot` operation to
`client` package.

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

netmap/netmap_test.go

@@ -0,0 +1,47 @@
+package netmap_test
+
+import (
+	"testing"
+
+	v2netmap "github.com/nspcc-dev/neofs-api-go/v2/netmap"
+	"github.com/nspcc-dev/neofs-sdk-go/netmap"
+	netmaptest "github.com/nspcc-dev/neofs-sdk-go/netmap/test"
+	"github.com/stretchr/testify/require"
+)
+
+func TestNetMapNodes(t *testing.T) {
+	var nm netmap.NetMap
+
+	require.Empty(t, nm.Nodes())
+
+	nodes := []netmap.NodeInfo{netmaptest.NodeInfo(), netmaptest.NodeInfo()}
+
+	nm.SetNodes(nodes)
+	require.ElementsMatch(t, nodes, nm.Nodes())
+
+	nodesV2 := make([]v2netmap.NodeInfo, len(nodes))
+	for i := range nodes {
+		nodes[i].WriteToV2(&nodesV2[i])
+	}
+
+	var m v2netmap.NetMap
+	nm.WriteToV2(&m)
+
+	require.ElementsMatch(t, nodesV2, m.Nodes())
+}
+
+func TestNetMap_SetEpoch(t *testing.T) {
+	var nm netmap.NetMap
+
+	require.Zero(t, nm.Epoch())
+
+	const e = 158
+
+	nm.SetEpoch(e)
+	require.EqualValues(t, e, nm.Epoch())
+
+	var m v2netmap.NetMap
+	nm.WriteToV2(&m)
+
+	require.EqualValues(t, e, m.Epoch())
+}