bundle2: record processing results in the bundleoperation object Part handlers can now add records to the `bundleoperation` object. This can be used to help other parts or to let the caller of the unbundling process react to the results.

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.

bundle2: feed a unbundle20 to the `processbundle` function The unbundle can comes from multiple sources. (on disk file, peer, etc) and (ultimately) of multiple type (bundle10, bundle20). The `processbundle` is no longer in charge of creating the bundle.