Lower timeout for make_membership_event

Calls to make_membership_event are done in response to client requests,
and so should not be retried over long timeframes.

synapse/federation/transport/client.py

@@ -179,7 +179,8 @@ class TransportLayerClient(object):
         content = yield self.client.get_json(
             destination=destination,
             path=path,
-            retry_on_dns_fail=True,
+            retry_on_dns_fail=False,
+            timeout=20000,
         )
 
         defer.returnValue(content)