routing+routerrpc: add capacity in rpcs

FetchPairCapacity is used by the following endpoints to introduce the
capacity for probability calculations:

* QueryProbability
* QueryRoutes

routing/mock_graph_test.go

@@ -226,6 +226,31 @@ func (m *mockGraph) fetchNodeFeatures(nodePub route.Vertex) (
 	return lnwire.EmptyFeatureVector(), nil
 }
 
+// FetchAmountPairCapacity returns the maximal capacity between nodes in the
+// graph.
+//
+// NOTE: Part of the routingGraph interface.
+func (m *mockGraph) FetchAmountPairCapacity(nodeFrom, nodeTo route.Vertex,
+	amount lnwire.MilliSatoshi) (btcutil.Amount, error) {
+
+	var capacity btcutil.Amount
+
+	cb := func(channel *channeldb.DirectedChannel) error {
+		if channel.OtherNode == nodeTo {
+			capacity = channel.Capacity
+		}
+
+		return nil
+	}
+
+	err := m.forEachNodeChannel(nodeFrom, cb)
+	if err != nil {
+		return 0, err
+	}
+
+	return capacity, nil
+}
+
 // htlcResult describes the resolution of an htlc. If failure is nil, the htlc
 // was settled.
 type htlcResult struct {