By default, `SELECT FOR UPDATE` will also lock any rows which are
`JOIN`ed into the selected rows; in the case of UserMessage rows, this
can mean arbitrary Message rows.
Since the messages themselves are not being changed, it is not
necessary to lock them -- and doing so may lead to deadlocks, in the
case that the UserMessage row is locked for update before the Message,
and some other request has already taken a read lock on the Message
and is blocked on the UserMessage write lock.
Change `select_for_update_query` to explicitly only lock UserMessage.
Alex Vandiver
Anders Kaseorg