Add operation to clone storage handles and tokens

Add "clone" operation to duplicate storage handles and tokens.

This allows the "run retcon as a server" function to duplicate, e.g., the PostgreSQL connection and give the API server thread and the retcon processing thread their own connection.

Please don't delete the branch after merging. :-)
This doesn't really explain much to me, what's "a handle" in this case, and what does duplication mean? The e.g. just confuses me further.