The grouping logic in `do_delete_messages` calls
`message.is_stream_message()` in a loop, which needs to access
message.recipient. This is obviously super inefficient if .recipient
hasn't been prefetched for the message objects.
`delete_in_topic` is the only function that calls `do_delete_messages`
with many messages, so this is the only spot we need to fix, to address
the immediate bug.
Of course a better improvement would be to fix `do_delete_messages` to
do something smarter than naively accessing message.recipient in a loop.
f119c33789