lightningd: expose created_index and updated_index fields.

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>

lightningd/invoice.h

@@ -39,6 +39,8 @@ struct invoice_details {
 	u8 *features;
 	/* The offer this refers to, if any. */
 	struct sha256 *local_offer_id;
+	/* Index values */
+	u64 created_index, updated_index;
 };
 
 /**