Provide an initializing servlet for servlet 2.3 web containers that call servlet init before initializing listeners

Some servlet 2.3 application servers call Servlet.init() before publishing a ContextInitialized event for servlet listeners. This generates a large amount of logging from servlets expecting a Spring context to be available at a well known servlet context attribute as this attribute is set by a listener upon receiving the ContextInitialized event.

It would appear that Confluence will still recover from this (at least the DWR servlet does) but it would be nice to have proper support for this lifecycle ordering. We probably need to use an initializing servlet like Spring's ContextLoaderServlet when operatiing in these web containers.

The servlet 2.4 specification requires listeners to be initialized first so this problem will not affect containers implementing this spec.