Issue #2491155 by mikeburrelljr, Mac_Weber, opdavies: Update drupal.org and kernel.org URLs in core modules (Follow-up to 2489912)

Issue #2471737 by klausi: PageCache::get() should return FALSE on a cache miss as promised by the docs

Issue #2368987 by Wim Leers, Berdir, Schnitzel, epari.siva, bircher, vijaycs85, likin: Move internal page caching to a module to avoid relying on config get on runtime