renaming project, product or series breaks vcs imports

Importd spreads jobs on slaves by hashing the job name, which is of the form [project-]product-series. Since projects, products and series can be renamed, and their associations can change, the slave assigned to a RCS import job can change.

Since job migration would currently require manual operation, some ``undocumented features'' in Importd, related to Arch archive registration, have not been fixed since they effectively provide early failure in the cases where a manual job migration needs to be done.

This the reason why renaming or reassociating ProductSeries associated with a RCS import are strongly discouraged. They generally break the RCS import.