channeldb: add subscription to control tower

Allows other sub-systems to subscribe to payment success and fail
events.

routing/mock_test.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"sync"
 
+	"github.com/go-errors/errors"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/htlcswitch"
 	"github.com/lightningnetwork/lnd/lntypes"
@@ -284,3 +285,9 @@ func (m *mockControlTower) FetchInFlightPayments() (
 
 	return fl, nil
 }
+
+func (m *mockControlTower) SubscribePayment(paymentHash lntypes.Hash) (
+	bool, chan PaymentResult, error) {
+
+	return false, nil, errors.New("not implemented")
+}