Issue #2931443 by Wim Leers, casey, zuuperman, drpal: Browser parsing race condition: JSON.parse sometimes called on partially streamed BigPipe placholder replacements, causes fatal JS error

Issue #2723705 by Wim Leers, catch: Reduce the 200ms BigPipe placeholder interval to 50ms and make it overridable

Issue #2670444 by Wim Leers: Embedded AJAX responses aren't application/json, but application/vnd.drupal-ajax

Issue #2469431 by Wim Leers, Fabianx, swentel, nod_, yched, dawehner, tstoeckler, catch, borisson_: BigPipe for auth users: first send+render the cheap parts of the page, then the expensive parts