This is consistent with our output changes, and increases consistency.
It also keeps future sanity checks happy, that we only use JSON msat
helpers with '_msat' fields.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Changed: JSON-RPC: `invoice`, `sendonion`, `sendpay`, `pay`, `keysend`, `fetchinvoice`, `sendinvoice`: `msatoshi` argument is now called `amount_msat` to match other fields.
Changelog-Deprecated: JSON-RPC: `invoice`, `sendonion`, `sendpay`, `pay`, `keysend`, `fetchinvoice`, `sendinvoice` `msatoshi` (use `amount_msat`)
