performance: Avoid Recipient lookup for stream messages.

All the fields of a stream's recipient object can
be inferred from the Stream, so we just make a local
object.  Django will create a Message object without
checking that the child Recipient object has been
saved.  If that behavior changes in some upgrade,
we should see some pretty obvious symptom, including
query counts changing.

Tweaked by tabbott to add a longer explanatory comment, and delete a
useless old comment.

zerver/tests/test_message_send.py

@@ -1194,7 +1194,7 @@ class StreamMessagesTest(ZulipTestCase):
                 body=content,
             )
 
-        self.assert_length(queries, 14)
+        self.assert_length(queries, 13)
 
     def test_stream_message_dict(self) -> None:
         user_profile = self.example_user('iago')