Review servlet web server factories being ResourceLoaderAware

[wilkinsona]

The servlet web server factories have been ResourceLoaderAware since they were introduced:

• Jetty
• Tomcat
• Undertow

Each factory uses the ResourceLoader, if set, to obtain the class loader that it uses. We should review this, with a couple of questions in mind:

• Should the ClassLoader be provided in a way that doesn't add setResourceLoader to the factories' public API?
• Should the reactive factories be updated to use the same class loading arrangement?