Don't target unassigned storages for reconciliation

To support variable replication factor, repository host node assignments were introduced in 243dc385. Repository's assignments indicate which storages are supposed to contain a copy of the repository. Since only the assigned storages need to contain the repository, the reconciler should take the assignments in to account.

This commit changes the reconciler to not target outdated, unassigned repositories for reconciliation. They may still act as the source node in order to facilitate moving repositories to different storages.

While the unassigned repositories are not targeted for now, in the future the reconciler will delete them. This is left for later when the functionality is more mature.

The functionality is behind a toggle as Praefect doesn't yet fully support variable replication factor. The functionality can later be enabled with per repository elector once the rest of the functionality has been implemented.

Part of #3154 (closed)