Make CSS/JS aggregation work for parallel rendering

If you have parallel rendering of different page elements, then there is not one single PHP process to either:

1. hold a static of all assets.

2. collect stuff from all the render arrays on the page via #attached.

This means that sub-requests/blocks that require CSS or JavaScript (or want to add an RSS feed to the HTML head or whatever) are out of luck.

We discussed this at length at DrupalCon Munich and BADCamp, and currently have a draft of the following:

- all out-of-band stuff, all of them, need to use #attached, see #1839338: [meta] Remove drupal_set_*() drupal_add_*() in favour of #attached/response for out-of-band stuff

- #attached needs to work with the response object - probably we need a parallel or replacement implementation for things that return those instead of render arrays.

- when using a 'real' reverse proxy or SSI we came up with two approaches for getting the assets from the sub-requests to the main page:

1. All nested blocks need to declare the assets they need up front via a method, without having their render method called. So that the parent page can collect the dependencies without rendering the blocks. This would mean replacing #attached and having a method on block controllers or similar. However it's going to run into serious problems with something like FAPI where the form element might use #attached and the parent block callback can't know what's in there.

2. Everything uses #attached, then when rendering the block, we add some JSON to the HTML of that page. We then have a small script loader that finds assets in the JSON and adds them to the page so they're loaded asynchronously (or with RSS feeds/page title changes just adds/changes the HTML as required). This introduces a js dependency if you're using real sub-requests but it'd be pluggable in case someone comes up with something better.

For 'fake' reverse proxy, i.e. HttpCache, we could do the processing in the parent PHP process - so execute the sub-request, collect the #attached stuff, figure it out from there - very similar to how drupal_render() handles it now. The metadata for either approach is the same, it's just the processing of it by the 'parent' that's different.