Add an automatic stub provider and initializer for XWikiContext

The goal is to make sure the ExecutionContext automatically contains a usable XWikiContext even for deamon thread. When ExecutionContextInitializer is called the first time for a request, the valid XWikiContext is used to create a stub XWikiContext and then each ExecutionContextInitializer called from daemon thread with no XWikiContext already in the ExecutionContext a clone of the stub XWikiContext is injected in it.