Datasources on the new bus

Splitting up datasources and policy engines to communicate over a bus requires that each policy engine get the entire snapshot of the datasource after the policy engine subscribes to the datasource, and then apply updates to the snapshot to stay in sync with the datasource.

The pub-sub channel does not provide the sender a way to control who receives each update, so the sender cannot coordinate sending the snapshot first and then following with the updates. Instead the sender can only send updates continuously and send a snapshot out of band. To give the recipient a way to know when to start applying updates to the snapshot, the sender must assign sequence numbers to each update, and also assign a sequence number to the snapshot.

This communication pattern called the "Clone" pattern in the zeromq manual.
See http://zguide.zeromq.org/php:chapter5#Reliable-Pub-Sub-Clone-Pattern