bundle2: introduce a bundleoperation object This object will hold all data and state gathered through the processing of a bundle. This will allow: - each handler to be aware of the things unbundled so far - the caller to retrieve data about the execution - bear useful object and logic (like repo, transaction) - bear possible bundle2 reply triggered by the unbundling. For now the object is very simple but it will grow at the same time as the bundle2 implementation.