routing: improve equal cost route comparison

When the (virtual) payment attempt cost is set to zero, probabilities
are no longer a factor in determining the best route. In case of routes
with equal costs, we'd just go with the first one found. This commit
refines this behavior by picking the route with the highest probability.
So even though probability doesn't affect the route cost, it is still
used as a tie breaker.

routing/pathfind.go

@@ -493,13 +493,25 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 			int64(cfg.PaymentAttemptPenalty),
 		)
 
-		// If the current best route is better than this candidate
-		// route, return. It is important to also return if the distance
-		// is equal, because otherwise the algorithm could run into an
-		// endless loop.
+		// If there is already a best route stored, compare this
+		// candidate route with the best route so far.
 		current, ok := distance[fromVertex]
-		if ok && tempDist >= current.dist {
-			return
+		if ok {
+			// If this route is worse than what we already found,
+			// skip this route.
+			if tempDist > current.dist {
+				return
+			}
+
+			// If the route is equally good and the probability
+			// isn't better, skip this route. It is important to
+			// also return if both cost and probability are equal,
+			// because otherwise the algorithm could run into an
+			// endless loop.
+			probNotBetter := probability <= current.probability
+			if tempDist == current.dist && probNotBetter {
+				return
+			}
 		}
 
 		// Every edge should have a positive time lock delta. If we