implement expired tokens deletion

Things that I would really like a review on: (but all comments are welcome!)

• Timestamp format: I assumed that timestamps in the DB are NTP seconds. Is that correct? If so, could it be added as a comment to the Entity file?
• This MR adds a dependency to robert-server-common (to use a time utility function). Is that OK? If yes, any policy about tracking versions?
• Should we do bulk deletion? (I remember reading a caveat about spring lifecycle methods not being used in that case...)
• Any comments regarding test refactoring?

Thanks!