Make the Locking API generic

With D7 we should def. be able to clock on lock-types, not only nodes. So a touple (type, object-id, user-id), not only (nid, uid). This is needed a lot, actually also for Drupal 6 so we can lock on comments, Taxonomy-editing or whatever people think about. This should become a more general api of "checking object / checkout", so we have a much wider use-case.