routing: add sufficient link capacity to our relaxation condition

This commit modifies our modified version of Dijkstra's to include
sufficient link capacity within the algorithm’s relaxation condition.
With this change, we’ll now avoid exploring the graph in the direction
of a link that has insufficient capacity, allowing us to terminate path
finding more quickly.

routing/pathfind.go

@@ -331,7 +331,9 @@ func findRoute(graph *channeldb.ChannelGraph, target *btcec.PublicKey,
 			// TODO(roasbeef): add capacity to relaxation criteria?
 			//  * also add min payment?
 			v := newVertex(edge.Node.PubKey)
-			if tempDist < distance[v].dist {
+			if tempDist < distance[v].dist &&
+				edgeInfo.Capacity >= amt {
+
 				distance[v] = nodeWithDist{
 					dist: tempDist,
 					node: edge.Node,