htlcswitch: sync link hand-off

This commit extends the link with a new synchronous delivery point for
local UpdateAddHTLC messages. The switch method SendHTLC is updated to
use this delivery point and thereby becomes a synchronous call.

For MPP payments, synchronous hand-off is important. Otherwise the next
pathfinding round could start without the channel balance updated yet.

htlcswitch/mock.go

@@ -702,6 +702,11 @@ func (f *mockChannelLink) HandleSwitchPacket(pkt *htlcPacket) error {
 	return nil
 }
 
+func (f *mockChannelLink) HandleLocalAddPacket(pkt *htlcPacket) error {
+	_ = f.mailBox.AddPacket(pkt)
+	return nil
+}
+
 func (f *mockChannelLink) HandleChannelUpdate(lnwire.Message) {
 }