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/interfaces.go

@@ -70,6 +70,10 @@ type ChannelLink interface {
 	// possible).
 	HandleSwitchPacket(*htlcPacket) error
 
+	// HandleLocalAddPacket handles a locally-initiated UpdateAddHTLC
+	// packet. It will be processed synchronously.
+	HandleLocalAddPacket(*htlcPacket) error
+
 	// HandleChannelUpdate handles the htlc requests as settle/add/fail
 	// which sent to us from remote peer we have a channel with.
 	//