Store SWHIDs removed since the graph has been exported
When handling takedown notices, we can remove SWHIDs from the archive. Ideally swh.graph
would store the list of objects removed since the export. It would help for #4770 but we could also use the list to prevent swh.graph
methods from returning non-existent nodes.
This should not be an append-only list of identifiers that have been removed: objects could also be restored after their initial removal.