No event if a Paid Invoice gets rolled back

[raffecat]

We need to send an event if it turns out (unlikely but possible) that we have marked an invoice as paid
and sent an event, but then a payment to the invoice gets rolled back due to a fork.

[xre217]

Fix: #158 — emits PAYMENT_UNCONFIRMED on reorg after confirm. Tips in DOGE: DTRJECKXfxgnSzGbt7TYRikLPDocqvdmo5