Read_only.SRead-only stores are store where it is only possible to read existing values.
val mem : [> Perms.read ] t -> key -> bool Lwt.tmem t k is true iff k is present in t.
val find : [> Perms.read ] t -> key -> value option Lwt.tfind t k is Some v if k is associated to v in t and None is k is not present in t.
val close : 'a t -> unit Lwt.tclose t frees up all the resources associated with t. Any operations run on a closed handle will raise Closed.