multi: add htlcNotifier interface to switch and link

In this commit, a htlcNotifier interface is added to allow for easy
unit testing. Instances of the HtlcNotifier are added to the server,
switch and link.

htlcswitch/link_test.go

@@ -1750,6 +1750,7 @@ func newSingleLinkTestHarness(chanAmt, chanReserve btcutil.Amount) (
 		MaxFeeAllocation:      DefaultMaxLinkFeeAllocation,
 		NotifyActiveChannel:   func(wire.OutPoint) {},
 		NotifyInactiveChannel: func(wire.OutPoint) {},
+		HtlcNotifier:          aliceSwitch.cfg.HtlcNotifier,
 	}
 
 	aliceLink := NewChannelLink(aliceCfg, aliceLc.channel)
@@ -4313,6 +4314,7 @@ func (h *persistentLinkHarness) restartLink(
 		MaxFeeAllocation:      DefaultMaxLinkFeeAllocation,
 		NotifyActiveChannel:   func(wire.OutPoint) {},
 		NotifyInactiveChannel: func(wire.OutPoint) {},
+		HtlcNotifier:          aliceSwitch.cfg.HtlcNotifier,
 	}
 
 	aliceLink := NewChannelLink(aliceCfg, aliceChannel)
@@ -5523,6 +5525,7 @@ func TestCheckHtlcForward(t *testing.T) {
 			},
 			FetchLastChannelUpdate: fetchLastChannelUpdate,
 			MaxOutgoingCltvExpiry:  DefaultMaxOutgoingCltvExpiry,
+			HtlcNotifier:           &mockHTLCNotifier{},
 		},
 		log:           log,
 		channel:       testChannel.channel,