htlcswitch: auto-fail held htlcs

Make the interceptable switch aware of htlc expiry and fail back htlcs in-time
to prevent the counterparty from force-closing the channel.

htlcswitch/interfaces.go

@@ -312,6 +312,10 @@ type InterceptedPacket struct {
 
 	// OnionBlob is the onion packet for the next hop
 	OnionBlob [lnwire.OnionPacketSize]byte
+
+	// AutoFailHeight is the block height at which this intercept will be
+	// failed back automatically.
+	AutoFailHeight int32
 }
 
 // InterceptedForward is passed to the ForwardInterceptor for every forwarded