routing: replace sort.Sort with sort.Slice in router.go

Use sort.Slice in FindRoutes function in routing/router.go, as part
of the move to use new language features. Remove sortableRoutes type
wrapper for slice of Routes since it is no longer needed to sort routes.

routing/router.go

@@ -869,7 +869,7 @@ func (r *ChannelRouter) FindRoutes(target *btcec.PublicKey,
 	// each path. During this process, some paths may be discarded if they
 	// aren't able to support the total satoshis flow once fees have been
 	// factored in.
-	validRoutes := make(sortableRoutes, 0, len(shortestPaths))
+	validRoutes := make([]*Route, 0, len(shortestPaths))
 	for _, path := range shortestPaths {
 		// Attempt to make the path into a route. We snip off the first
 		// hop in the path as it contains a "self-hop" that is inserted
@@ -894,7 +894,19 @@ func (r *ChannelRouter) FindRoutes(target *btcec.PublicKey,
 	// Finally, we'll sort the set of validate routes to optimize for
 	// lowest total fees, using the required time-lock within the route as
 	// a tie-breaker.
-	sort.Sort(validRoutes)
+	sort.Slice(validRoutes, func(i, j int) bool {
+		// To make this decision we first check if the total fees
+		// required for both routes are equal. If so, then we'll let the total time
+		// lock be the tie breaker. Otherwise, we'll put the route with the lowest
+		// total fees first.
+		if validRoutes[i].TotalFees == validRoutes[j].TotalFees {
+			timeLockI := validRoutes[i].TotalTimeLock
+			timeLockJ := validRoutes[j].TotalTimeLock
+			return timeLockI < timeLockJ
+		}
+
+		return validRoutes[i].TotalFees < validRoutes[j].TotalFees
+	})
 
 	log.Debugf("Obtained %v paths sending %v to %x: %v", len(validRoutes),
 		amt, dest, newLogClosure(func() string {