Multi-tier caching causes multiple test regressions

This revision implemented multi-tier caching (with memcache or arraycache in front of SQL cache)

https://github.com/civicrm/civicrm-core/pull/336

Which caused these tests to regress:

api_v3_SettingTest.testGetFieldsCaching
api_v3_SystemTest.testFlush
CRM_Utils_Cache_SqlGroupTest.testPrefetch

It seems reasonable that the 'flush' operation fails – in normal deployment, there are now additional copies of the data stored in memory (in local-static variables for CRM_Core_BAO_Cache::

Item

and in the singleton instance of CRM_Utils_Cache_ArrayCache). These structures don't appear to be flushed. However, there seems to be more to it: after various attempts at flushing those caches (crm-12321-attempt

.patch and a few other formulations), we still don't have a fix for SystemTest.testFlush().