If you miss a wait event, you can catch up by doing listinvoices and
getting the max of these fields. It's also a good debugging clue.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
This makes it easier to access (rather than decoding bolt11).
Changelog-Added: JSON-RPC: `invoice` now outputs explicit `payment_secret` it its own field.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>