Define procedure for retention of deleted data on pillars

As we have decided not to implement undo delete/replace functionality in the protocol, we should define a procedure for the pillar operation organization to retaining the deleted files for a periode of time after the data has been deleted from the bit repository.

A critical component in this is of course the ability to import data back into the pillar in case of a wish to restore data. As a consequence of this we also need to be able to reestablish data integrity after the direct ingestion (BITMAG-278).