Clean up of json files

[rmader]

The json-files we use for the AAA system currently never get cleaned up, making them grow forever and saving user data (like ips) in clear text.

As I need some kind of cleaup system for the DAO.authentication file anyways, I'd like to implement a cleanup function (has to be thread-save) that removes all entries with a valid_until key that holds a timestamp smaller then the current one.
It has to be triggered by a thread that wakes up maybe every ten minutes or so, triggering the cleanup for the corresponding files.

Edit: to be future save, we should use long as the timestamp format.

[Orbiter]

We should not store data in the AAA files which then have to be cleaned up. This effect is circumvented with dc57121 where volatile information is never stored in the json files.