Enable or disable async mode

[oliemansm]

After the refactor of async mode support the possibility to configure async support has been removed entirely. This should be reinstated, while still keeping the new way of checking it using the HttpServletRequest itself, e.g.

public ServletRegistrationBean dispatcherServlet() {
    ServletRegistrationBean registration = new ServletRegistrationBean(new DispatcherServlet(), "/");
    registration.setAsyncSupported(true);
    return registration;
}

The old property should be made available again too, but now it will be true by default. So to disable it you'd have to add the following property:

graphql.servlet.async-mode-enabled: false