routing+routerrpc: add multiple outgoing channel restriction

lnrpc/routerrpc/router_backend_test.go

@@ -159,7 +159,7 @@ func testQueryRoutes(t *testing.T, useMissionControl bool, useMsat bool) {
 			t.Fatal("unexpected last hop")
 		}
 
-		if *restrictions.OutgoingChannelID != outgoingChan {
+		if restrictions.OutgoingChannelIDs[0] != outgoingChan {
 			t.Fatal("unexpected outgoing channel id")
 		}