Op.Wait_filesWaiting on files.
FIXME now that we have Futs maybe we can maybe get rid of this.
val v_op : id:id -> mark:mark -> created:B00_std.Time.span -> ?post_exec:(op -> unit) -> ?k:(op -> unit) -> B00_std.Fpath.t list -> opv declares a wait files operation, these are stored in reads.
val v : unit -> tv constructs a bare wait files operation.