When I Export and Import a Space, I should get the full History

(User Story, Investigate , Priority: High, Test Status: No automated tests yet , Reported By Justin du Coeur, )
Import and Export, History
Summary: Huge change, but we should completely rewrite the usual export/import protocol and format, to be History-based.
That is, instead of exporting the snapshot view of the Space, we should export its full History.
This is a complete rewrite, but the resulting code will likely be much more robust, since important then becomes almost identical to normal Space loading.
It is not clear that this export format should be XML. I can make a good case that we should just export the Kryo-compressed data, precisely the same as the Cassandra table. Indeed, in a sense this is nothing but exporting the Cassandra table for this Space.
We probably want to retain the XML export and import functions, but those will then be focused on external export. Backing up a Space and moving it somewhere else becomes all about the History. (Indeed, we might call this feature "backup/restore" vs. "export/import".)