[MERGE] Apicalypse: remove the old API

This drops the backward compatibility with the old API:
 - remove method wrappers;
 - convert all the remaining old-API calls to the new API;
 - remove the processing of _columns, _defaults, _inherit_fields, _all_columns;
 - remove all old-API fields;
 - implement sparse fields;
 - modify the registry to become a mapping model_name -> model_class;
 - adapt XML-RPC and JSON-RPC gateways.