make triple-sure that the future scheduler is properly shutdown (see also 702278df3b57);
--- a/src/Pure/System/session.ML Mon Feb 10 22:08:18 2014 +0100
+++ b/src/Pure/System/session.ML Mon Feb 10 22:22:06 2014 +0100
@@ -57,6 +57,7 @@
Outer_Syntax.check_syntax ();
Future.shutdown ();
Event_Timer.shutdown ();
+ Future.shutdown ();
session_finished := true);