Implement rigorous locking in HiveWhiteBoard

• Hide data to-be-protected inside a Synced template which guarantees locking prior to access, and which provides an explicit scope for the lock.
• Replace _CALL macro with a fwd function which forwards the argument to the current Partition. Automatically obtain the relevent service from the Partition (including locking) and then invoke the callback.
• Provide a generic for_ which calls the provided callback for each Partition, after locking it.