Since we know the total reservations on each hop, we can more easily
determine probabilities than using flowset_probability() which has to
replicate this collision detection.
We leave both in place for now, to check. The results are not
identical, due to slightly different calculation methods.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
askrene.c was getting quite long, and this is self-contained.
The only code change is a convenience accessor for the per-htlc-cost
hash table.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>