Issue #2657684 by Wim Leers, Fabianx, xjm, effulgentsia: Refactor BigPipe internals to allow a contrib module to extend BigPipe with the ability to stream anonymous responses and prime Page Cache for subsequent visits

Issue #2712935 by Wim Leers, Fabianx, catch, alexpott: Messages are not rendered last, meaning those messages set within placeholders only appear on the next request

Issue #2678662 by Wim Leers, mondrake, Fabianx, alexpott: Ensure BigPipe does not break when HTML document contains CDATA sections or inline scripts matching certain patterns

Issue #2698811 by danmuzyka, Wim Leers, jhedstrom, Fabianx: BigPipe unnecessarily renders and sends multiple-occurrence placeholders multiple times when using JS — can cause JS errors

Issue #2702001 by olafkarsten, Wim Leers: Inject config factory for BigPipe (including in exception handlers)

Issue #2665992 by alexpott, klausi, heykarthikwithu, xjm, tstoeckler: @file is not required for classes, interfaces and traits

Issue #2678568 by Wim Leers: Ensure good UX & DX even when A) rendering of placeholder fails, B) some response event subscriber fails

Revert "Issue #2678568 by Wim Leers: Ensure good UX & DX even when A) rendering of placeholder fails, B) some response event subscriber fails"

This reverts commit eee35007.

Issue #2678568 by Wim Leers: Ensure good UX & DX even when A) rendering of placeholder fails, B) some response event subscriber fails

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