htlcswitch: add notifications for forwards

This commit adds notifications for htlcs which are forwarded through
our node. Forwards are notified when the htlc is added on our ougoing
link, settles when we send a settle message to the downstream peer.
If a failure occurs, we check whether it occurred at our node, then
notify a link or forwarding failure accordingly.

Note that this change also adds forward event notifications for sends
which are initiated by our node because the handling code for adding
a htlc which originates from our node is the same as that for handling
forwards. Htlcs for our locally initiated sends have our internal pid
set in the incoming htlcs id field, so we extract this value and notify
with a zero htlc id to be consistent with receives (which have zero
outgoing circuits). Subsequent settles or failures are not noitfied
for local sends in this commit, and will be handled in a follow up.

htlcswitch/link.go

@@ -1418,6 +1418,18 @@ func (l *channelLink) handleDownStreamPkt(pkt *htlcPacket, isReProcess bool) {
 
 		l.cfg.Peer.SendMessage(false, htlc)
 
+		// Send a forward event notification to htlcNotifier.
+		l.cfg.HtlcNotifier.NotifyForwardingEvent(
+			newHtlcKey(pkt),
+			HtlcInfo{
+				IncomingTimeLock: pkt.incomingTimeout,
+				IncomingAmt:      pkt.incomingAmount,
+				OutgoingTimeLock: htlc.Expiry,
+				OutgoingAmt:      htlc.Amount,
+			},
+			getEventType(pkt),
+		)
+
 	case *lnwire.UpdateFulfillHTLC:
 		// If hodl.SettleOutgoing mode is active, we exit early to
 		// simulate arbitrary delays between the switch adding the
@@ -1476,6 +1488,12 @@ func (l *channelLink) handleDownStreamPkt(pkt *htlcPacket, isReProcess bool) {
 		l.cfg.Peer.SendMessage(false, htlc)
 		isSettle = true
 
+		// Send a settle event notification to htlcNotifier.
+		l.cfg.HtlcNotifier.NotifySettleEvent(
+			newHtlcKey(pkt),
+			getEventType(pkt),
+		)
+
 	case *lnwire.UpdateFailHTLC:
 		// If hodl.FailOutgoing mode is active, we exit early to
 		// simulate arbitrary delays between the switch adding a FAIL to
@@ -1529,10 +1547,28 @@ func (l *channelLink) handleDownStreamPkt(pkt *htlcPacket, isReProcess bool) {
 		htlc.ChanID = l.ChanID()
 		htlc.ID = pkt.incomingHTLCID
 
-		// Finally, we send the HTLC message to the peer which
-		// initially created the HTLC.
+		// We send the HTLC message to the peer which initially created
+		// the HTLC.
 		l.cfg.Peer.SendMessage(false, htlc)
 		isSettle = true
+
+		// If the packet does not have a link failure set, it failed
+		// further down the route so we notify a forwarding failure.
+		// Otherwise, we notify a link failure because it failed at our
+		// node.
+		if pkt.linkFailure != nil {
+			l.cfg.HtlcNotifier.NotifyLinkFailEvent(
+				newHtlcKey(pkt),
+				newHtlcInfo(pkt),
+				getEventType(pkt),
+				pkt.linkFailure,
+				false,
+			)
+		} else {
+			l.cfg.HtlcNotifier.NotifyForwardingFailEvent(
+				newHtlcKey(pkt), getEventType(pkt),
+			)
+		}
 	}
 
 	// If this newly added update exceeds the min batch size for adds, or