is_new_best is always false for updates

[xivh]

If there are conflicting configs during casm update, the one that is chosen does not get "is_new_best" : true in the report. This also happens in update_config_unstable. This means that if you calculate some new configs and they end up mapping back to existing ones, the properties of the existing ones could be updated and you wouldn't know by looking at the reports.

It might also be nice to have a previous_best.