[meta] Drupal 8 Cache API improvements

Opening this meta issue to keep track of the various patches trying to modernize Drupal 8's cache API.

Done:

Clean up cache API to match interface: #81461: Clean up the cache API (stop overloading function arguments, remove procedural wrappers)
Remove bc layer: #1272706: Remove backwards compatibility layer for cache API.

In progress:

Cache tags: #636454: Cache tag support
PSR-0 re-org: #1323120: Convert cache system to PSR-0
Add setMultiple(): #1167248: Add CacheBackendInterface::setMultiple()
Decouple from hook_schema() #1167144: Make cache backends responsible for their own storage.
Better handling of invalid/expired cache entries #1774332: Better handling of invalid/expired cache entries

TODOs/postponed on above

Rip out CACHE_TEMPORARY and use cache tags instead #730060: Replace CACHE_TEMPORARY, cache_clear_all() and minimum cache lifetime with cache tags support.

Remove prefix clears and use cache tags instead #1393392: Convert prefix cache clears to cache tags, then remove support for them

Evaluate PSR cache object interface when it's decided: (http://groups.google.com/group/php-standards/browse_thread/thread/b0b3e1...)

Make the default bin 'default' instead of 'cache'. (no issue yet).

Major bugs affecting D7

:
CACHE_TEMPORARY and cron #891600: Page and block caches are cleared the same way on cron as on content change

cache_lifetime and all bins being cleared: #1015946: Eliminate $user->cache and {session}.cache in favor of $_SESSION['cache_expiration'][$bin].