routing+channeldb: findPath uses Vertex instead of LightningNode

This commit modifies the nodeWithDist struct to use a route.Vertex
instead of a *channeldb.LightningNode. This change, coupled with
the new ForEachNodeChannel function, allows the findPath Djikstra's
algorithm to cut down on database lookups since we no longer need
to call the FetchOtherNode function.

routing/heap.go

@@ -3,6 +3,7 @@ package routing
 import (
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/lnwire"
+	"github.com/lightningnetwork/lnd/routing/route"
 )
 
 // nodeWithDist is a helper struct that couples the distance from the current
@@ -12,9 +13,9 @@ type nodeWithDist struct {
 	// current context.
 	dist int64
 
-	// node is the vertex itself. This pointer can be used to explore all
-	// the outgoing edges (channels) emanating from a node.
-	node *channeldb.LightningNode
+	// node is the vertex itself. This can be used to explore all the
+	// outgoing edges (channels) emanating from a node.
+	node route.Vertex
 
 	// amountToReceive is the amount that should be received by this node.
 	// Either as final payment to the final node or as an intermediate