[#323] cmd/node: Compose functions for closing components in app

Closing callback can be registered in app through onShutdown method.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

cmd/neofs-node/main.go

@@ -78,12 +78,15 @@ func wait(c *cfg) {
 }
 
 func shutdown(c *cfg) {
-	c.cfgGRPC.server.GracefulStop()
-	c.cfgControlService.server.GracefulStop()
-
-	c.log.Info("gRPC server stopped")
+	for _, closer := range c.closers {
+		closer()
+	}
 
 	goOffline(c)
 
 	c.wg.Wait()
 }
+
+func (c *cfg) onShutdown(f func()) {
+	c.closers = append(c.closers, f)
+}