PersistenceStrategy to assign string based keys to objects persisted in files. The file
naming strategy is based on the key's type name and its toString method. It escapes non
digit, non a-z and A-Z characters. In order to change the escaping/unescaping algorithm,
simply extend this class and rewrite its getName/extractKey methods. Note, this
implementation silently implies that the keys actually are Strings, since the keys will be
turned into string keys at deserialization time.