SnapshotTail
class SnapshotTail @InternalWowApi constructor(val snapshot: SnapshotRecord?, val tail: AggregateTail)
What SnapshotTailReader.readWithSnapshot reads of one aggregate: its snapshot, null when it has none, and the tail after it, every stream of the aggregate when there is no snapshot.
Fields may be added: construct one with of; the constructor is internal to Wow.