bundle2: support for bundling and unbundling payload We add the ability to bundle and unbundle a payload in parts. The payload is the actual binary data of the part. It is used to convey all the applicative data. For now we stick to very simple implementation with all the data fit in single chunk. This open the door to some bundle2 testing usage. This will be improved before bundle2 get used for real. We need to be able to stream the payload in multiple part to exchange any changegroup efficiently. This simple version will do for now. Bundling and unbundling are done in the same changeset because the test for parts is less modular. However, the result is not too complex.