[Robustness] The system must support reordering of events

[isabelcosta]

• Related to Advanced Features #5
• Since the central server is updated opportunistically, users can update the central server with information that depends on events that are still not in the central server.
• For example: user A cycles to gain points and then sends all the points to user B. Now user B updates the central server before user A reports that it gained points by cycling. When such situations occur, the system must be able to:
  • i) accept the points given to user B
  • ii) update te score of user A
• The server must always know how the users earned their points