Ignore new messages when lists are behind.

If individual messages arrive before we get the latest
messages from the server, they can create gaps in rendering,
and would often be offscreen anyway, so we just ignore them.

static/js/fetch_status.js

@@ -45,6 +45,10 @@ var FetchStatus = function () {
         return !loading_newer && !found_newest;
     };
 
+    self.can_append = function () {
+        return found_newest;
+    };
+
     return self;
 
 };