https://github.com/lightningnetwork/lightning-rfc/pull/877 talks about
removing this restriction (only Electrum actually enforced it on
receive), so start by allowing creation of giant invoices, though
we mark them as requiring mpp.
Changelog-Changed: JSON-RPC: `invoice` now allows creation of giant invoices (>= 2^32 msat)
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>