rework state-saving

[rjbs]

From #26:

So we actually want send_reminder to do the removal of the item, but that means that check-reminders might run twice and find the same reminder in the queue, not yet removed by send_reminder, which is still waiting. So we probably want the yield to send_reminder-then-save-state to mark the job as taken in such a way that it won't be saved to state, but will prevent second-enqueueing of the send-reminder.