Retry if the heartbeat connection dies (#5066)

Otherwise it will keep trying to write to the broken connection and
memory will leak because the event dispatcher will keep appending
the message to _outbound_buffer.

celery/worker/heartbeat.py

@@ -46,6 +46,7 @@ class Heart(object):
                                  active=len(active_requests),
                                  processed=all_total_count[0],
                                  loadavg=load_average(),
+                                 retry=True,
                                  **SOFTWARE_INFO)
 
     def start(self):