demote "iopub lacks parent" to debug-level

these happen all the time, and aren’t worth warning about

IPython/parallel/controller/hub.py

@@ -845,7 +845,7 b' class Hub(SessionFactory):'
 
         parent = msg['parent_header']
         if not parent:
-            self.log.warn("iopub::IOPub message lacks parent: %r", msg)
+            self.log.debug("iopub::IOPub message lacks parent: %r", msg)
             return
         msg_id = parent['msg_id']
         msg_type = msg['header']['msg_type']