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/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