lwcapi: fix push for new subs (#1684)

If subs change after the data source has been pushed, then
it wouldn't push the high priority messages until the next
pull. Now it will go ahead and push if messages are ready.

atlas-lwcapi/src/main/scala/com/netflix/atlas/lwcapi/WebSocketSessionManager.scala

@@ -89,7 +89,7 @@ private[lwcapi] class WebSocketSessionManager(
           } else {
             // Only pull when no push happened, because push should have triggered a pull
             // from downstream
-            pull(in)
+            onPull()
           }
         }
       }