Retire an alias on a date, with a replacement

Give an alias deprecatedAt and replacedBy (another alias or a model). Until the date its responses carry ib-alias-deprecated: <date>; replaced-by=<target> and the request's Logs page shows it as deprecated; from the date its requests run as the replacement, recorded on the receipt as a migration, or are refused with a 400 naming the date when there is none.