htlcswitch: add new ticker in switch to batch log forwarding events

In this commit, we extend the switch as is, to record details
concerning settled payment circuits. To do this, we introduce a new
interface to the package: the ForwardingLog. This is a tiny interface
that simply lets us abstract away the details of the storage backing of
the forwarding log.

Each time we receive a successful HTLC settle, we’ll log the full
details (chans, fees, time) as a pending forwarding log entry. Every 15
seconds, we’ll then batch flush out these entries to disk. When we’re
exiting, we’ll try to flush out all entries to ensure everything gets
recorded to disk.

htlcswitch/mock.go

@@ -80,6 +80,22 @@ func (m *mockFeeEstimator) Stop() error {
 
 var _ lnwallet.FeeEstimator = (*mockFeeEstimator)(nil)
 
+type mockForwardingLog struct {
+	sync.Mutex
+	events map[time.Time]channeldb.ForwardingEvent
+}
+
+func (m *mockForwardingLog) AddForwardingEvents(events []channeldb.ForwardingEvent) error {
+	m.Lock()
+	defer m.Unlock()
+
+	for _, event := range events {
+		m.events[event.Timestamp] = event
+	}
+
+	return nil
+}
+
 type mockServer struct {
 	started  int32
 	shutdown int32