Add support for memcached

CiviCRM currently support only memcache: http://php.net/manual/en/book.memcache.php which is a bit older and not maintained. In PHP 5.4 support might be dropped

Hence we also need to support memcached: http://php.net/manual/en/book.memcached.php

Since they use the same set of config parameters, we will not change those (and hence preserve compatibility). To use memcached we need to use:

define( 'CIVICRM_USE_MEMCACHE', 'memcached' );