[router] Calc min-to-node fees based on min value, not est value

[router] Calc min-to-node fees based on min value, not est value

When walking the network graph to calculate a route, we always
calculate the minimum fee which is required to make one further
hop. This avoids some extra hop processing at the end of each path
selection loop (saving around 10% runtime in our benchmarks).

However, if we use the real value which we expect
to send over a channel in that calculation, we may find an
alternate path to the same node which is more expensive but
capacity-constrained, resulting in us considering it cheaper as the
relative fee paid will be lower.

Instead, we can use the `minimal_value_contribution_msat`, which is
a constant through an entire path finding iteration, as the amount,
preventing any basis change in the relative fee paid.