routing: make shardHandler aware of payment session

This commit adds payment session to shardHandler to enable private edge
policies being updated in shardHandler. The relevant interface and mock
are updated. From now on, upon seeing a ChannelUpdate message,
shardHandler will first try to find the target policy in additionalEdges
and update it. If nothing found, it will then check the database for
edge policy to update.

routing/mock_test.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"sync"
 
+	"github.com/btcsuite/btcd/btcec"
 	"github.com/go-errors/errors"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/htlcswitch"
@@ -167,6 +168,18 @@ func (m *mockPaymentSession) RequestRoute(_, _ lnwire.MilliSatoshi,
 	return r, nil
 }
 
+func (m *mockPaymentSession) UpdateAdditionalEdge(_ *lnwire.ChannelUpdate,
+	_ *btcec.PublicKey, _ *channeldb.ChannelEdgePolicy) bool {
+
+	return false
+}
+
+func (m *mockPaymentSession) GetAdditionalEdgePolicy(_ *btcec.PublicKey,
+	_ uint64) *channeldb.ChannelEdgePolicy {
+
+	return nil
+}
+
 type mockPayer struct {
 	sendResult    chan error
 	paymentResult chan *htlcswitch.PaymentResult