renepay bug fix: list previous sendpays

[Lagrang3]

I tried renepay on mainnet. At first the command timed-out, hence I called it
again and then I've got

Strange error from sendpay: {"code":201,"message":"There already is a payment with payment_hash=..., groupid=2, partid=1. Either change the partid, or wait for the payment to complete and start a new group."}{
   "code": -4,
   "message": "Plugin terminated before replying to RPC call."
}

While investigating this problem I've found a couple of bugs on the function that processes the result of listsendpays and decides the new groupid and the start counter for partid. Also shadow fees were buggy.
This PR fixes those.

[vincenzopalazzo]

Concept ACK for the C side, but I have a small comment on the python test

[cdecker]

This PR was subsumed in your other PR, so this is closed for being empty :-)