1. Hoist 7200 constant into the bolt12 heade2.
2. Make preimage the last createinvoice arg, so we could make it optional.
3. Check the validity of the preimage in createinvoice.
4. Always output used flag in listoffers.
5. Rename wallet offer iterators to offer_id iterators.
6. Fix paramter typos.
7. Rename `local_offer_id` parameter to `localofferid`.
8. Add reference constraints on local_offer_id db fields.
9. Remove cut/paste comment.
10. Clarify source of fatal() messages in wallet.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
