Flush attributes as soon as they are set

This is inefficient in terms of network traffic, but it means governors
are saved immediately, which is usually what the user wants.

client/attribute.cpp

@@ -352,6 +352,9 @@ void attribute_set(int key, int id, int x, int y, size_t data_length,
   } else {
     attribute_hash->remove(akey);
   }
+
+  // Sync with the server
+  attribute_flush();
 }
 
 /**