Changes:
* Fields renumbered to their draft values + billion.
* offer_recurrence now comes in compulsory or optional (backwards compat) flavors.
* `proportional_amount` is now inside `offer_recurrence_base` not `offer_recurrence_paywindow`.
* New field `invreq_recurrence_cancel`.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-EXPERIMENTAL: Draft specification for recurring offers changed: old recurring offers will no longer work.
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