askrene-getroutes: fix the value of the amount and delay

[Lagrang3]

In the previous implementation getroutes would provide for every i-th hop the value of the amount and delay which correspond to the (i-1)-th HTLC. This commit changes that, instead the i-th hop contains the value of the amount and delay of the i-th HTLC.

Changelog-Changed: askrene: getroutes outputs contains for each hop the correct amount and delay values.

Previously proposed as #7639

[rustyrussell]

I appreciate this is better for how we use the routes, but we can't break the API.

[rustyrussell]

If we were to make this change, what is the fee for this route? What is the total delay? You can no longer tell? Information has been lost.

We can't change the API anyway, but we could do:

1. Add "node_id" with the prev node id (to make that explicit)
2. Add "amount_msat_for_next" and "delay_for_next" which would be these fields.
3. Make the documentation describe these fields more clearly.

[Lagrang3]

Agree, we could add those fields instead of changing the API.
Or we could add for each route (not each hop) a total_delay and total_amount_msat, values.

Or maybe leave getroutes as it is and create a new rpc, for instance askrene-routes, just any different name,
that gives us precisely the quantities that we need to feed to the sphinx_path API.

For example:

deliver 1000 to C, final_cltv = 10
A -> B -> C
base fee = 1
delay delta = 6

routes: [
    {total_amount: 1002,
     final_amount: 1000,
     total_delay: 22,
     final_delay: 10,
     final_node: C,
     path: [ {node: A, channel: AB, amount: 1001, delay: 16},
                  {node: B, channel: BC, amount: 1000, delay: 10}] }
]

To build the onions:

for each route:
    sp = new sphinx_path()
    for each hop in route['path']:
        payload = onion_nonfinal_hop(hop['channel'], hop['amount'], hop['delay']+blockheight)
        sp.add_hop(hop['node'], payload)
    sp.add_final_hop(route['final_node'], route['final_amount'], ...)