routing: findRoute now returns a slice of selected hops in reverse order

This commit modifies the findRoute function to decouple the
validation+creation of a route, from the path finding algorithm itself.
When we say “route”, we mean the full payment route complete with
time-lock and fee information. When we say “path” we simple mean an
ordered set of channel edges from one node to another target node.

With this commit we can now perform path finding independent of route
creation which will be needed in the up coming refactor to implement a
new modified k-shortest paths algorithm.

routing/router.go

@@ -1052,7 +1052,10 @@ func (r *ChannelRouter) ProcessRoutingMessage(msg lnwire.Message, src *btcec.Pub
 
 // FindRoute attempts to query the ChannelRouter for the "best" path to a
 // particular target destination which is able to send `amt` after factoring in
-// channel capacities and cumulative fees along the route.
+// channel capacities and cumulative fees along the route.  Once we have a set
+// of candidate routes, we calculate the required fee and time lock values
+// running backwards along the route. The route that will be ranked the highest
+// is the one with the lowest cumulative fee along the route.
 func (r *ChannelRouter) FindRoute(target *btcec.PublicKey, amt btcutil.Amount) (*Route, error) {
 	dest := target.SerializeCompressed()
 
@@ -1067,14 +1070,24 @@ func (r *ChannelRouter) FindRoute(target *btcec.PublicKey, amt btcutil.Amount) (
 		return nil, ErrTargetNotInNetwork
 	}
 
+	// First we'll find a single shortest path from the source (our
+	// selfNode) to the target destination that's capable of carrying amt
+	// satoshis along the path before fees are calculated.
+	//
 	// TODO(roasbeef): add k-shortest paths
-	route, err := findRoute(r.cfg.Graph, r.selfNode, target, amt)
+	routeHops, err := findRoute(r.cfg.Graph, r.selfNode, target, amt)
 	if err != nil {
 		log.Errorf("Unable to find path: %v", err)
 		return nil, err
 	}
 
-	// TODO(roabseef): also create the Sphinx packet and add in the route
+	// If we were able to find a path we construct a new route which
+	// calculate the relevant total fees and proper time lock values for
+	// each hop.
+	route, err := newRoute(amt, routeHops)
+	if err != nil {
+		return nil, err
+	}
 
 	log.Debugf("Obtained path sending %v to %x: %v", amt, dest,
 		newLogClosure(func() string {