Load a literal based on the value, language and type passed as argument. The method will first look in the node cache for cached nodes. If no cache entry is found, it will run a database query ("load.literal_by_v") on the NODES table and construct a new KiWiLiteral using the values of the literal columns (svalue, ivalue, ...). The type of literal returned depends on the value of the ntype column.
When a node is loaded from the database, it will be added to the different caches to speed up subsequent requests.
@param value string value of the literal to load
@param lang language of the literal to load (optional, 2-letter language code with optional country)
@param ltype the type of the literal to load (optional)
@return the literal matching the given arguments or null if it does not exist
@throws SQLException