finalize proofs earlier to reduce memory requirement;

--- a/src/Pure/proofterm.ML	Fri Jul 26 09:50:23 2019 +0200
+++ b/src/Pure/proofterm.ML	Fri Jul 26 09:59:11 2019 +0200
@@ -1660,7 +1660,7 @@
     else
       (singleton o Future.forks)
         {name = "Proofterm.fulfill_proof_future", group = NONE,
-          deps = Future.task_of body :: map (Future.task_of o snd) promises, pri = 0,
+          deps = Future.task_of body :: map (Future.task_of o snd) promises, pri = 1,
           interrupts = true}
         fulfill
   end;