session: support the expiration of private tokens

All sessions in NeoFS has limited in epochs lifetime. There is a need
to limit the lifetime of private session tokens.

This commmit:

  * extends PrivateToken interface with Expired method;

  * defines EpochLifetimeStore interface with RemoveExpired method
    and embeds it to PrivateTokenStore interface;

  * adds epoch value parameter to private token constructor.

session/store_test.go

@@ -9,7 +9,7 @@ import (
 
 func TestMapTokenStore(t *testing.T) {
 	// create new private token
-	pToken, err := NewPrivateToken()
+	pToken, err := NewPrivateToken(0)
 	require.NoError(t, err)
 
 	// create map token store
@@ -33,3 +33,64 @@ func TestMapTokenStore(t *testing.T) {
 	// ascertain that returned token equals to initial
 	require.Equal(t, pToken, res)
 }
+
+func TestMapTokenStore_RemoveExpired(t *testing.T) {
+	// create some epoch number
+	e1 := uint64(1)
+
+	// create private token that expires after e1
+	tok1, err := NewPrivateToken(e1)
+	require.NoError(t, err)
+
+	// create some greater than e1 epoch number
+	e2 := e1 + 1
+
+	// create private token that expires after e2
+	tok2, err := NewPrivateToken(e2)
+	require.NoError(t, err)
+
+	// create token store instance
+	s := NewMapTokenStore()
+
+	// create storage keys for tokens
+	id1, err := refs.NewUUID()
+	require.NoError(t, err)
+	id2, err := refs.NewUUID()
+	require.NoError(t, err)
+
+	assertPresence := func(ids ...TokenID) {
+		for i := range ids {
+			_, err = s.Fetch(ids[i])
+			require.NoError(t, err)
+		}
+	}
+
+	assertAbsence := func(ids ...TokenID) {
+		for i := range ids {
+			_, err = s.Fetch(ids[i])
+			require.EqualError(t, err, ErrPrivateTokenNotFound.Error())
+		}
+	}
+
+	// store both tokens
+	require.NoError(t, s.Store(id1, tok1))
+	require.NoError(t, s.Store(id2, tok2))
+
+	// ascertain that both tokens are available
+	assertPresence(id1, id2)
+
+	// perform cleaning for epoch in which both tokens are not expired
+	require.NoError(t, s.RemoveExpired(e1))
+
+	// ascertain that both tokens are still available
+	assertPresence(id1, id2)
+
+	// perform cleaning for epoch greater than e1 and not greater than e2
+	require.NoError(t, s.RemoveExpired(e1+1))
+
+	// ascertain that tok1 was removed
+	assertAbsence(id1)
+
+	// ascertain that tok2 was not removed
+	assertPresence(id2)
+}