MDL-31501: Adding Memcached session handler

Add $CFG->session_save_path_memcached to your config.php.  This
value is used for the session.save_path, which is documented
here: http://www.php.net/manual/en/memcached.sessions.php

Session handling can be configured via PHP ini settings
that are documented here:

http://www.php.net/manual/en/memcached.configuration.php
MDL-31501 session: Move memcached_session to core\session namespace
MDL-31501 session: Drivers must handle kill, gc, touch and friends
MDL-31501 session: Renaming the legacyfile driver to file
MDL-31501 session: More documentation in config-dist.php