The name in the spec is `msat`, but I don't want to make an API exception.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Added: JSON-RPC: `fetchinvoice` `changes` `amount_msat`
Changelog-Deprecated: JSON-RPC: `fetchinvoice` `changes` `msat` (use `amount_msat`)