maintain deprecated \OC::$session when getting or setting the session via the server container or UserSession

restore order os OC::$session and OC::$CLI

remove unneded initialization of dummy session

write back session when $useCustomSession is true

log warning when deprecated app is used

[error] 4461#0: *186285 FastCGI sent in stderr: "PHP message
: PHP Fatal error:  Call to a member function execute() on null in /var/www/ownc
loud/cron.php on line 125" while reading response header from upstream, client:
217.13.183.252, server: cloud.mycloud.com, request: "GET /cron.php HTTP/1.1", upstre
am: "fastcgi://unix:/var/run/php5-fpm.sock:", host: "cloud.mycloud.com", referrer: "
https://cloud.mycloud.com/"

Fixes #2012

Typo in comment "feature" => "future"

now should work with temp files too

respect coding style

Add "cron.php" for background jobs. It is named cron.php because more people will recognize the purpose of the file then.