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/switch.go

@@ -491,7 +491,7 @@ func (s *Switch) SendHTLC(firstHop lnwire.ShortChannelID, paymentID uint64,
 		return linkErr
 	}
 
-	return link.HandleSwitchPacket(packet)
+	return link.HandleLocalAddPacket(packet)
 }
 
 // UpdateForwardingPolicies sends a message to the switch to update the