Ignore backoff history for invites, aliases, and roomdirs

Add a param to the federation client which lets us ignore historical backoff
data for federation queries, and set it for a handful of operations.

synapse/handlers/directory.py

@@ -175,6 +175,7 @@ class DirectoryHandler(BaseHandler):
                         "room_alias": room_alias.to_string(),
                     },
                     retry_on_dns_fail=False,
+                    ignore_backoff=True,
                 )
             except CodeMessageException as e:
                 logging.warn("Error retrieving alias")