This update completes a rough round trip from taking an initial snapshot to restoring it.
There is still some polishing work to be done here!

Things that still need doing:

1) Currently not snapshot content items are being written to the database as they are transferred to the bridge.
2) There is no verification step at this point in time.
3) Hooks for logging state transitions within spring batch should be handled by step listeners - currently state
   changes are being logged at the end of the job - so currently not as granular as it will need to be.
4) Error handling for failed batch writers is incomplete: still need to investigate the best way to handle.
5) Unit tests for SnapshotExecutionListener do not cover restore operations.
6) SnapshotExecutionListener should probably be refactored into two listener classes: one for restores, one for snapshots.