Log some bits about event creation (#4121)

I found these helpful in debugging my room upgrade tests.

synapse/handlers/message.py

@@ -427,6 +427,9 @@ class EventCreationHandler(object):
 
         if event.is_state():
             prev_state = yield self.deduplicate_state_event(event, context)
+            logger.info(
+                "Not bothering to persist duplicate state event %s", event.event_id,
+            )
             if prev_state is not None:
                 defer.returnValue(prev_state)