bundle2: lazy unbundle of part payload The `unbundle` part gains a `read` method to retrieve payload content. This method behaves as a python file-like read method. The bundle-processing code is updated to make sure a part is fully consumed before another one is extracted. Test output changes because the debug output is even more interleaved now.

util: support None size in chunkbuffer.read() When no size is provided, read the whole buffer. This aligns with the usual behavior of `read()` in python.

bundle2: lazily iterate over bundle parts in the test We used to create a list to know the number of parts in the bundle. This prevents any lazy reading as planned for real usage. The list creation is dropped. Some test output changed as debug output is now interleaved with command output.