use PersistentMapping as selection storage if the site is configured as using memcached but no memcached plugin found. this workaround is required for updating sites where memcached storage are already used.


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@27762 20353a03-c40f-0410-a6d1-a30d3c3de9de