TicketHolder statistics are spread across multiple structures and methods: QueueStats (_holderStats, _exemptStats), _delinquencyStats, and separate appendHolderStats, appendTicketStats, and appendExemptStats.

Introduce a single stats class that owns all queue, ticket, exempt, and delinquency metrics, and expose a unified API for updates and BSON serialization. This will simplify maintenance, reduce duplication, and make stats easier to extend.