Spoolable.Metadata.t should be smallish since it is read and written more
frequently than Spoolable.Data.t.
module Data = DataSpoolable.Data.t is where the "real" data lives and it allows for data-specific
load and save functionality.
module Queue = QueueQueue.t is an enumerable type that represents the available queues and the
mapping to directory names on-disk.
module Name_generator : Async_smtp__.Multispool_intf.Name_generator.S with type t = Async_smtp_types.Smtp_envelope.t and module Unique_name = Id