MDL-31501 rework user session architecture
List of changes: * New OOP API using PHP namespace \core\session\. * All handlers now update the sessions table consistently. * 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. * Workaround for memcached version with non-functional gc. * Improved security - forced session id regeneration. * Improved compatibility with recent PHP releases. * 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. * Fast purging of sessions used in request only. * No legacy distinction - file, database and memcached support the same functionality. * Session handler name included in performance info. * Fixed user_loggedin and use...