renepay: fees seems to be off

@Lagrang3 arbeitet bereits daran.

Seit 08.2.2024.

I tried a payment (see the command line output below), the notes suggest that the total fees will amount to `600msat`, but the difference between the amount sent and receive is actually `121378msat`. Notice also that the flow amount `60009778msat` does not correspond to the reported `amount_sent_msat`.

```
$ cln renepay lnbc...
# we have computed a set of 1 flows with probability 0.982, fees 598msat and delay 446
# Flow 1: amount=60009778msat prob=0.982 fees=600msat delay=224 path=-826839x1256x0/0(min=max=6654910632msat)->-809546x2028x1/1->-800968x722x1/0->
# Flow 1: Success
{
"payment_preimage": "...",
"payment_hash": "ff8c13f5a126e17fd90fdba5b58c91195966e6bedb15f85297159f361303835f",
"created_at": 1707412130.071523515,
"parts": 1,
"amount_msat": 59889000,
"amount_sent_msat": 60010378,
"status": "complete",
"destination": "03d607f3e69fd032524a867b288216bfab263b6eaee4e07783799a6fe69bb84fac"
}
```