Craft cacheMethod setting is being overridden
The craft/config/memcache.php file has nothing to do with where PHP sessions are stored.  It is for Craft's general purpose data caching.

If you want to save PHP session in memcache, you'll need to set the php.ini save_handler and save_path similar to: https://www.digitalocean.com/community/tutorials/how-to-store-php-sessions-in-memcached-on-a-centos-vps

At this point, I'd suggest not using the overridePhpSessionLocation config setting.  Everything it does can be done through php.ini and it's been removed from the upcoming Craft 3 release anyway.