Earlier, we were using 'send_event' & 'queue_json_publish' in
'do_send_messages' which can lead to a situation where we enqueue
events but the transaction fails at a later stage.
Events should not be sent until we know we're not rolling back.
For the common case of not needing to reference the UserMessage row
later, and for being a stream without private history, the UserMessage
row is irrelevant. Convert `has_user_message` to a thunk, and defer
loading it unless necessary.
