ReservationKey
class ReservationKey @InternalWowApi constructor(val namespace: String, val key: String, val system: Boolean = false)
A reserved or released key in namespace, without its owner or expiry: what an append records. Its scope follows from the entry's aggregate (ReservationScope.of): its type and its tenant, or no tenant when the key is a system one, unique across tenants.
Fields may be added: construct one with of; the constructor is internal to Wow.