Client Synchronization: Ensure multiple same synchronizations don't crash

[Zhiyuan-Amos]

If synchronization occurs multiple times, then the same data will be CUD. This can lead to errors if not handled well. Let's:

1. Add conditional logic to ignore C & D if the entry already exists or does not exist respectively.
2. Update only if the incoming data is newer (assuming Client DateTime are synchronized)

To prevent race conditions with local CUD. local CUD should be disallowed when synchronization is running.