Faster cache for get_joined_hosts

synapse/federation/transaction_queue.py

@@ -187,6 +187,8 @@ class TransactionQueue(object):
                             prev_id for prev_id, _ in event.prev_events
                         ],
                     )
+                    destinations = set(destinations)
+                    logger.info("destinations: %r", destinations)
 
                     if send_on_behalf_of is not None:
                         # If we are sending the event on behalf of another server