Auto unsubscribe on after last tx + workaround orphan records

- Replication subscriber now automatically terminate subscription after processing the last subscription transaction. Before the subscription was terminated by a timeout.
- Workaround orphaned records. An orphan record is possible when all the parent versions are GC for a given replicated timestamp (i.e. all remaining parent versions exist at a later timestamp). The workaround force the publisher to use the consistent store as the replication data source instead of the transaction log.