Issue #2935256 by Jo Fitzgerald, voleger, kim.pepper, ibustos, Yogesh Pawar, catch, jibran: Remove all usages of drupal_get_message and drupal_set_message in modules

Issue #2863607 by Lendude, naveenvalecha, Wim Leers, Jo Fitzgerald: Convert WebTestBaseTests of BigPipe to BrowserTestBase

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 #2567715 by marthinal, hgoto, jpd4nt, mr.baileys, Wim Leers, alexpott: #markup generated by #pre_render callbacks do not get processed by ensureMarkupIsSafe()

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 #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 #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