sendonion RPC does not allow to set the total amount in lightningd's
wallet, therefore it mixing sendpay and sendonion payment parts would
not work. That means for the time being we cannot complete a payment
initialized with sendpay until we add a total_amount parameter to
sendonion.
Changelog-None.
Signed-off-by: Lagrang3 <lagrang3@protonmail.com>