AggregateTail
class AggregateTail @InternalWowApi constructor(val streams: List<EventStreamRecord>, val earlier: List<CommittedRequest>)
What EventLog.readTail reads of one aggregate in one round trip: its streams from a version on, in version order, and the request keys of its streams before that version, the latest first (earlier).
Fields may be added: construct one with of; the constructor is internal to Wow.