routing: during path finding skip edge if amount < edge.MinHTLC

In this commit, we update path finding to skip an edge if the amount
we’re trying to route through it is below the MinHTLC (in mSAT) value
for that node. We also add a new test to exercise this behavior. In
order for out test to work properly, we’ve modified the JSON to make
the edge to Goku have a higher min HTLC value.

routing/pathfind.go

@@ -490,12 +490,11 @@ func findPath(tx *bolt.Tx, graph *channeldb.ChannelGraph,
 			// record the new better distance, and also populate
 			// our "next hop" map with this edge. We'll also shave
 			// off irrelevant edges by adding the sufficient
-			// capacity of an edge to our relaxation condition.
+			// capacity of an edge and clearing their min-htlc
+			// amount to our relaxation condition.
 			if tempDist < distance[v].dist &&
-				edgeInfo.Capacity >= amt.ToSatoshis() {
-
-				// TODO(roasbeef): need to also account
-				// for min HTLC
+				edgeInfo.Capacity >= amt.ToSatoshis() &&
+				amt >= outEdge.MinHTLC {
 
 				distance[v] = nodeWithDist{
 					dist: tempDist,
@@ -518,6 +517,8 @@ func findPath(tx *bolt.Tx, graph *channeldb.ChannelGraph,
 				heap.Push(&nodeHeap, distance[v])
 			}
 
+			// TODO(roasbeef): return min HTLC as error in end?
+
 			return nil
 		})
 		if err != nil {

routing/testdata/basic_graph.json

@@ -64,7 +64,7 @@
             "channel_point": "89dc56859c6a082d15ba1a7f6cb6be3fea62e1746e2cb8497b1189155c21a233:0", 
             "flags": 0, 
             "expiry": 1,
-            "min_htlc": 1, 
+            "min_htlc": 1000, 
             "fee_base_msat": 10, 
             "fee_rate": 1000, 
             "capacity": 100000