Credit goes to Eric Merrill, thanks!

Also changing course_backend.php coding style
according to codechecker.

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 user_loggedout event triggering.
 * Other minor bugfixing and improvements.
 * Fixed database session segfault if MUC disposed before $DB.

Limitations:
 * Session access time is now updated right after session start.
 * Support for $CFG->sessionlockloggedinonly was removed.
 * First request does not update userid in sessions table.
 * The timeouts may break badly if server hosting forces PHP.ini session settings.
 * The session GC is a lot slower, we do not rely on external session timeouts.
 * There cannot be any hooks triggered at the session write time.
 * File and memcached handlers do not support session lock acquire timeouts.
 * Some low level PHP session functions can not be used directly in Moodle code.

This is used by messaging system for sending/receiving message
to/from noreply or support user. message_send api will now use
core_user class to get noreply or support user and then
send/receive message depending on user state.

Includes:
* no more hacky reloads, everything is written only once and kept until cache reset
* lang menu list is now cached in MUC
* both string and lang menu caches are compatible with local caches on cluster nodes
* config-dist.php cleanup

MDL-40627 add 2G extramemorylimit option to admin UI and use the same units in config-dist.php to prevent confusion

Credit goes to Julien Boulen, thanks for spotting this.

Make "Advanced" and "Lockable" flags for any admin setting.

- 'List of courses' is split into 'List of courses' (available) and 'Enrolled courses', CFG->disablemycourses is deprecated;
- CFG->frontpageloggedin by default shows list of available courses;
- There is separate item to display course search box
- CFG->maxcoursesincombo is deprecated
- CFG->maxcategorydepth changed default value to 2 since we have AJAX loading now
- FRONTPAGECOURSELIMIT is transformed to CFG->frontpagecourselimit

c

Moving from phpunit test environment
to a specific behat test environment

Also more code comments and general cleanup

Includes X-Sendfile support, ETags for stored_files and general cleanup.