Don't create records in storage_repositories on generation increment
The storage_repositories rows should be created only with CreateRepository execution. We need to have more direct ways of managing database state with on disk state. The replication should happen only for truly existing storages. The repo existence check is now based on the database state after 7c517f28 (intercept RepositoryExists calls in Praefect, 2021-01-29), so the error 'pq: duplicate key value violates unique constraint "storage_repositories_pkey"' won't occur anymore.
Closes: #3183 (closed)