- introduce PaymentFeeBudget, which contains limits for fee budget and cltv budget
- when splitting a payment,
- the fee budget is linearly distributed between the parts
- this resolves a FIXME in lnrouter ("FIXME in case of MPP")
- the cltv budget is simply copied
- we could also add other kinds of budgets later, e.g. for the num in-flight htlcs
- resolves TODO in lnworker ("todo: compare to the fee of the actual route we found")