clarified order of shutdown;

--- a/src/Pure/PIDE/session.scala	Sun Jan 03 21:45:34 2016 +0100
+++ b/src/Pure/PIDE/session.scala	Mon Jan 04 20:23:14 2016 +0100
@@ -605,11 +605,11 @@
 
   def stop()
   {
+    delay_prune.revoke()
     manager.send_wait(Stop)
     prover.await_reset()
     change_parser.shutdown()
     change_buffer.shutdown()
-    delay_prune.revoke()
     manager.shutdown()
     dispatcher.shutdown()
   }