Ignore fees on first-hop channels from the public network graph

If we have a first-hop channel from a first-hop hint, we'll ignore
the fees on it as we won't charge ourselves fees. However, if we
have a first-hop channel from the network graph, we should do the
same.

We do so here, also teeing up a coming commit which will remove
much of the custom codepath for first-hop hints and start using
this common codepath as well.

Author: TheBlueMatt

lightning/src/routing/router.rs

@@ -2546,8 +2546,15 @@ where L::Target: Logger {
 						let curr_min = cmp::max(
 							$next_hops_path_htlc_minimum_msat, htlc_minimum_msat
 						);
-						let candidate_fees = $candidate.fees();
 						let src_node_counter = $candidate.src_node_counter();
+						let mut candidate_fees = $candidate.fees();
+						if src_node_counter == payer_node_counter {
+							// We do not charge ourselves a fee to use our own channels.
+							candidate_fees = RoutingFees {
+								proportional_millionths: 0,
+								base_msat: 0,
+							};
+						}
 						let path_htlc_minimum_msat = compute_fees_saturating(curr_min, candidate_fees)
 							.saturating_add(curr_min);