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/private_test.go

@@ -10,7 +10,7 @@ import (
 
 func TestPrivateToken(t *testing.T) {
 	// create new private token
-	pToken, err := NewPrivateToken()
+	pToken, err := NewPrivateToken(0)
 	require.NoError(t, err)
 
 	// generate data to sign
@@ -31,3 +31,20 @@ func TestPrivateToken(t *testing.T) {
 		),
 	)
 }
+
+func TestPToken_Expired(t *testing.T) {
+	e := uint64(10)
+
+	var token PrivateToken = &pToken{
+		validUntil: e,
+	}
+
+	// must not be expired in the epoch before last
+	require.False(t, token.Expired(e-1))
+
+	// must not be expired in the last epoch
+	require.False(t, token.Expired(e))
+
+	// must be expired in the epoch after last
+	require.True(t, token.Expired(e+1))
+}