Cleanup handling of temporary files

- Moves the logic of creating and cleaning up temporary files (and folders) from \OC_Helper to it's own class
- Create temp files with a prefix so we can recognize them
- Let cron cleanup any dangling temporary files

For #11401

cc @DeepDiver1975 @PVince81 @th3fallen 
no need for an sprintf here - the logger methods offer interpolation:
see: https://github.com/owncloud/core/blob/master/tests/lib/logger.php#L27

```php
		$logger->info('{Message {nothing} {user} {foo.bar} a}', array('user' => 'Bob', 'foo.bar' => 'Bar'));