Sync with HEAD changes and copy over changes.

Changed:
  * Fix for cache_clear_all issue in issue #199483.  Combined patches posted by Reg, firebus and robertDouglass.
  * Updated README.txt for memcache-session.inc

Added:
  * Added Drupal 5.0 - 5.6 patches for core serialization.
  * Added memcache-session.inc (working)