(!) Locking and/or transactions support. Safe adding of a new item with
pre-defined ID using checks that no such item already exists. Somehow
must deal with race condition if two attempts to add the same item are
made at the same time.

Fix destroy() on hash objects. It currently does not remove the object,
but empties it incorrectly.

Add `limit' option to searches to limit the number of results retrieved.

Negation of 'wq' and 'ws' in search() (Marcos' idea)

fill() method.

temporary_data() method to get per-object space to store something
temporary that would not be stored in the object on disk. Some cached
values or something like that. Just a hash or SimpleHash reference in
fact.
