Before
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* Requests for `CRM_Core_BAO_Cache` (`setItem($data,$group,$path)`,
  `getItem($group,$path)`, `getItems($group)`, `deleteGroup($group,$path)`)
  are *always* served by two tiers: (1) an in-memory array (`static::$cache`) and (2)
  an SQL table.

After
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* There is a config option `define('CIVICRM_BAO_CACHE_ADAPTER',
  'CRM_Core_BAO_Cache_Psr16');`.
    * When disabled (default), `CRM_Core_BAO_Cache` continues using the old code.
    * When enabled, `CRM_Core_BAO_Cache` changes behavior. Each `$group` is mapped to
      a PSR-16 object.

* The class/implementation for each `$group` depends on the configuration:
    * In a typical (non-Redis/non-Memcache) deployment, the implementation
      is `CRM_Utils_Cache_SqlGroup`, which has the same 2-tier structure
      (in-memory+SQL).
    * In Redis/Memcache deployment, the implementation combines
      `FastArrayDecorator` with `CRM_Utils_Cache_Redis` or
      `CRM_Utils_Cache_Memcache`.  This gives a similar 2-tier structure
      (e.g. in-memory+Redis).