Use snder and not event ID domain to check if ours

The transaction queue only sends out events that we generate. This was
done by checking domain of event ID, but that can no longer be used.
Instead, we may as well use the sender field.

synapse/federation/transaction_queue.py

@@ -175,7 +175,7 @@ class TransactionQueue(object):
                 def handle_event(event):
                     # Only send events for this server.
                     send_on_behalf_of = event.internal_metadata.get_send_on_behalf_of()
-                    is_mine = self.is_mine_id(event.event_id)
+                    is_mine = self.is_mine_id(event.sender)
                     if not is_mine and send_on_behalf_of is None:
                         return