routerrpc+htlcswitch: move intercepted htlc tracking to switch

In this commit we move the tracking of the outstanding intercepted htlcs
to InterceptableSwitch. This is a preparation for making the htlc
interceptor required.

Required interception involves tracking outstanding htlcs across
multiple grpc client sessions. The per-session routerrpc
forwardInterceptor object is therefore no longer the best place for
that.

lnrpc/routerrpc/router_server.go

@@ -890,7 +890,9 @@ func (s *Server) HtlcInterceptor(stream Router_HtlcInterceptorServer) error {
 	defer atomic.CompareAndSwapInt32(&s.forwardInterceptorActive, 1, 0)
 
 	// run the forward interceptor.
-	return newForwardInterceptor(s, stream).run()
+	return newForwardInterceptor(
+		s.cfg.RouterBackend.InterceptableForwarder, stream,
+	).run()
 }
 
 func extractOutPoint(req *UpdateChanStatusRequest) (*wire.OutPoint, error) {