[migractions] add sanity check in rename_relation_type If the relation is still present in the fs schema, we probably shouldn't be renaming it (because it's still in use somewhere, possibly in a different cube). Closes #3608172

[dataimport] Add safety belt on "gen_rtype" in MetaGenerator, closes #3712892 Some cubes may well add to the META_RTYPES set, hence crashing the gen_<somecubemeta> call.