XWIKI-9738: Finalize migration from workspaces to the new wiki API.

* Fixes.
XWIKI-9738: Finalize migration from workspaces to the new wiki API.

* Deleting the old workspace object at the very end of the migration, in order to be able to resume the migration when it is stopped.
Merge pull request #223 from gdelhumeau/XWIKI-9738

XWIKI-9738: Finalize migration from workspaces to the new wiki API.