thumbnail: Resolve a race condition when rendering messages.

Messages are rendered outside of a transaction, for performance
reasons, and then sent inside of one.  This opens thumbnailing up to a
race where the thumbnails have not yet been written when the message
is rendered, but the message has not been sent when thumbnailing
completes, causing `rewrite_thumbnailed_images` to be a no-op and the
message being left with a spinner which never resolves.

Explicitly lock and use he ImageAttachment data inside the
message-sending transaction, to rewrite the message content with the
latest information about the existing thumbnails.

Despite the thumbnailing worker taking a lock on Message rows to
update them, this does not lead to deadlocks -- the INSERT of the
Message rows happens in a transaction, ensuring that either the
message rending blocks the thumbnailing until the Message row is
created, or that the `rewrite_thumbnailed_images` and Message INSERT
waits until thumbnailing is complete (and updated no Message rows).

zerver/worker/thumbnail.py

@@ -170,7 +170,7 @@ def update_message_rendered_content(
                 message.rendered_content,
                 {} if image_data is None else {path_id: image_data},
                 {path_id} if image_data is None else set(),
-            )
+            )[0]
             if rendered_content is None:
                 # There were no updates -- for instance, if we re-run
                 # ensure_thumbnails on an ImageAttachment we already