Make default workflow_state 'unpublished' instead of nil (DDK-316)

nil has been interpreted as equivalent to 'unpublished', so this is a formal acknowledgment of that fact. nil is still technically allowed (i.e., workflow_state is defined as optional) in part for legacy data compatibility and for ease of use. Note that the publication service workflow (via the workflow gem) automatically translates nil to the initial workflow state, 'unpublished'.

This MR also expands the Ddr::V3 namespace to include new ChangeSet and ChangeSetPersister classes. Further development of these objects allow us to ease the transition from the original ChangeSet/Persister patterns with minimal disruption to existing processes.