Refine caching systems

CiviCRM includes several different mechanisms for caching. The internal APIs should be refactored for consistency/predictability/flexibility. The current caching approach includes:

• CRM_Core_BAO_Cache – SQL-backed caching
• CRM_Utils_Cache – Abstraction which supports in-process caching or memcache
• Static class variables and static function variables
• Cached per-user navigation in civicrm_setting.

There are several functions for cache flushing in CRM_Core_Config – as well as CRM_Utils_System::flushCache(), CRM_Core_Invoke::rebuildMenuAndCaches(), CRM_Utils_Cache_*, and civicrm_api3_system_flush.