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 
I understand your intention - but still no 3rdparty interfaces in our public api please - this has already been heavyly discussed as we introduced our ILogger interface - THX