[#259] pkg/netmap: Implement v2-compatible NetworkInfo type

Define NetworkInfo structure. Implement constructors, fields getters and
setters, binary and JSON encoding methods.

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

pkg/netmap/network_info_test.go

@@ -0,0 +1,53 @@
+package netmap
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestNetworkInfo_CurrentEpoch(t *testing.T) {
+	i := NewNetworkInfo()
+	e := uint64(13)
+
+	i.SetCurrentEpoch(e)
+
+	require.Equal(t, e, i.CurrentEpoch())
+	require.Equal(t, e, i.ToV2().GetCurrentEpoch())
+}
+
+func TestNetworkInfo_MagicNumber(t *testing.T) {
+	i := NewNetworkInfo()
+	m := uint64(666)
+
+	i.SetMagicNumber(m)
+
+	require.Equal(t, m, i.MagicNumber())
+	require.Equal(t, m, i.ToV2().GetMagicNumber())
+}
+
+func TestNetworkInfoEncoding(t *testing.T) {
+	i := NewNetworkInfo()
+	i.SetCurrentEpoch(13)
+	i.SetMagicNumber(666)
+
+	t.Run("binary", func(t *testing.T) {
+		data, err := i.Marshal()
+		require.NoError(t, err)
+
+		i2 := NewNetworkInfo()
+		require.NoError(t, i2.Unmarshal(data))
+
+		require.Equal(t, i, i2)
+	})
+
+	t.Run("json", func(t *testing.T) {
+		data, err := i.MarshalJSON()
+		require.NoError(t, err)
+
+		i2 := NewNetworkInfo()
+		require.NoError(t, i2.UnmarshalJSON(data))
+
+		require.Equal(t, i, i2)
+	})
+}