fee_setting: fix factor miscalulation bug

[3nprob]

Output is confusing when factors don't match up with actual ratio - I assume this is a bug?

Also moving down rounding to after calculations to get it correct.

[bitromortac]

Cool, thanks for spotting this. I think we should just guard against zero division errors. Perhaps something like:
factor_base_fee = base_fee_msat_new / base_fee_msat if base_fee_msat else float('inf')

[bitromortac]

Thank you!