Replace HtmlOutputProcessor by a safe API

[TimWolla]

HtmlOutputProcessor has issues due to state that needs to be preserved between ->process() and ->getHtml() and it also needs to modify external state (#4018).

Ideally the HtmlOutputProcessor would be deprecated in favor of a safe API that ensures that all the processing happens in a single atomic step and that does not provide any writable external fields during processing.

Care must be taken, because HtmlOutputNodeProcessor is intimately tied to HtmlOutputProcessor instead of some interface.