clarified messages streaming (again, amending 5ea3ed3c52b3): avoid too many small messages stacking up, e.g. when loading HOL-Analysis.Analysis.thy into Isabelle/jEdit;

--- a/src/Pure/Concurrent/consumer_thread.scala	Wed Dec 04 15:36:58 2019 +0100
+++ b/src/Pure/Concurrent/consumer_thread.scala	Wed Dec 04 19:40:22 2019 +0100
@@ -104,11 +104,7 @@
           }
         }
 
-        if (continue) {
-          val msgs1 = msgs.drop(reqs.length)
-          val msgs2 = mailbox.receive(timeout = Some(Time.zero))
-          main_loop(msgs1 ::: msgs2)
-        }
+        if (continue) main_loop(msgs.drop(reqs.length))
         else robust_finish()
     }