class type blockmap_client_t =object..end
method access : id:int -> identity:string -> size:int64 -> bool * Nn_blockmap.blockmap_tid and for datastore identity,
with an initial size.
The bool return flag is true if the blockmap has just been created
as empty structure and needs now to be populated.
method inactivate : id:int -> unitmethod shutdown : unit -> unitmethod close : unit -> unit