Handle getting cached async content with JSX with an AJAX request

The Async rendering framework currently memorized the call perform to the skinx module to use any on-demand resources (jsx, ssx, etc). The idea is that the async rendering is then able to return in an HTTP headers those resources references so they can be loaded when the data are available.
Now when the data is cached, the Async rendering framework directly calls back the skinx module methods while computing the content: this works perfectly well when computing a content to be displayed immediately. But it does not work properly when computing this content for an AJAX request, since in that case we'd want to know which JSX to load when getting the data.

The easiest solution, would be to be able to specify to Async rendering framework that we want a placeholder, even if the data is already available. Then once the placeholder retrieve through the AJAX request and put in the page, async.js would handle it to retrieve the data and the associated JSX.