Issue #2459819 by Wim Leers, rteijeiro: Remove CacheableInterface (and no longer let block plugins implement it)

Issue #2459155 by alexpott, pfrenssen, hussainweb, neclimdul: Remove REQUEST_TIME from bootstrap.php

Issue #2444231 by Wim Leers, nlisgo: Fix CacheableInterface so it makes sense (was: "Make Config objects & Entities implement CacheableInterface + add BubbleableMetadata::createFromCacheableObject()")

Issue #2457887 by prateekMehta, stefan.r, rpayanm, alexpott: Use Utility\SafeMarkup class instead of Utility\String for placeholder(), checkPlain(),format() functions

Issue #2451679 by Wim Leers, dawehner: Validate cache contexts (+ cache contexts in some views plugins wrong)

Issue #2428703 by Wim Leers: Add a 'user.permissions' cache context (was: "Should cache contexts be able to associate a cache tag?")

Issue #2411689 by alexpott: Use a MemoryBackend in StorageComparer so that configuration import validators don't have to reread data from disk or the db

Issue #2458413 by Wim Leers: BlockViewBuilder should specify cache contexts even for uncacheable blocks

Issue #2432837 by Wim Leers, Fabianx: Make cache contexts hierarchical (e.g. 'user' is more specific than 'user.roles')

Issue #2443073 by Wim Leers, joshtaylor: Add #cache[max-age] to disable caching and bubble the max-age

Issue #2433595 by chx, Berdir: WebTestBase::refreshVariables only resets cache_tags.invalidator.checksum

Issue #2329101 by Wim Leers: CacheableInterface only has a getCacheKeys() method, no getCacheContexts(), leads to awkward implementations

Issue #2318437 by Wim Leers, Gábor Hojtsy, geertvd: Replace the hardcoded langcode key on blocks with the 'language' cache context

Issue #2358079 by pwolanin, Berdir: Require a specific placeholder format in db_query() in order to trigger argument expansion, and require explicit 'IN' parameter for conditions

Issue #2040135 by Wim Leers, Berdir: Caches dependent on simple config are only invalidated on form submissions

Issue #2386161 by Berdir, Wim Leers, beejeebus, olli: ChainedFastBackend doesn't set 'expires' and 'tags' when writing to fastBackend

Issue #2328111 by dawehner, martin107, neclimdul: Replace most instances of the DRUPAL_ROOT constant with the app.root container parameter.

Issue #2340667 by YesCT, Pedro Lozano, alexpott, martin107: Protect Drupal\Core\Language\Language::id, and use getId().

Issue #2340123 by Wim Leers: Setting cache tags can be tricky: use strings instead of nested arrays to improve DX.

Issue #2332427 by neclimdul: Fixed APCUBackend makes incorrect assumptions about apc_fetch return type.

Issue #2248767 by effulgentsia, beejeebus, alexpott: Use fast, local cache back-end (APCu, if available) for low-write caches (bootstrap, discovery, and config).