Dummy blinded payment path followups #4326

#4152 added initial support for dummy blinded payment paths, but actually isn't giving us any privacy. Specifically, the dummy hops have a 0 CLTV and 0 fee. Instead, we really need to equalize the CLTV deltas at each hop (probably setting them all to at least whatever the maximum commonly used value is on the network, IIRC a few above 100), and also set reasonable(y low) fees on the dummy hops.

We also need tests covering attempts to underpay the dummy hop fees.