MDL-31501 rework user session architecture

List of changes:
 * New OOP API using PHP namespace \core\session\.
 * Experimental DB session support in Oracle.
 * Full support for session file handler (filesystem locking required).
 * New option for alternative session directory.
 * Official memcached session handler support.
 * Improved security.
 * Improved compatibility with recent PHP releases.
 * Session access time is now updated when starting session.
 * Fixed borked CSS during install in debug mode.
 * Switched to file based sessions in new installs.
 * DB session setting disappears if DB does not support sessions.
 * DB session setting disappears if session handler specified in config.php.
 * Other minor bugfixing and improvements.