Set of queries to deal with invoices. A couple of things to take into
account:
- Because the queries are not rewritten at runtime, we cannot have a
generic `INSERT` with different tuples.
- Because the queries are not rewritten at runtime, we cannot build
one query with only the filters that matter for that queries. The
two options are a combinatorial approach (a new query for every
permutation) or a generic query using the pattern
```
SELECT *
FROM table
WHERE (
-- Can be read as:
-- Match the filter 1 value if filter_1 != nil
column_1 >= sqlc.narg('filter_1') OR
sqlc.narg('filter_1') IS NULL
) AND (
column_2 >= sqlc.narg('filter_2') OR
sqlc.narg('filter_2') IS NULL
) ...
```
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)`.
