Updated netty version to 4.0.21.Final

To use:

```java
@WebListener
public class AppContextListener implements ServletContextListener {
    @Override
    public void contextDestroyed(final ServletContextEvent servletContextEvent) {
		
        try {
            FastThreadLocal.removeAll();
            FastThreadLocal.destroy();
        } catch (Exception e) {
            logger.error("Fatal error: {}", e);
        }
    }
}
```