This is the schema for "ordinal" BOLT11 invoices.
The invoices table aims to keep an entry for each invoice, BOLT11 or not,
that will be supported.
Invoice related HTLCs will be stored in a separete table than forwarded
htlcs.
SQLite does not support `SEQUENCE`. We achieve atomic autoincrementals
using primary keys with autoincrement/serial. An invoice `AddIndex`
translates to `invoices(id)` while `SettleIndex` is `invoice_payments(id)`.
