bundle2: add a ui argument to readbundle The bundle2 unbundler needs a ui argument. We are now passing this information to `readbundle`.

bundle2: move `readbundle` into the `exchange` module The `readbundle` function is going to understand the bundle2 header. We move the function to a more suitable place before making any other changes.

bundle2: use headerless HG10UN stream in changegroup Using `readbundle` in the part handlers creates a circular import hell. We are now using a simple `HG10UN` stream with no header. Some parameters may later be introduced on the part to change parameter. Producers are updated as well.

bundle2: allow using bundle2 for push We now support bundle2 for local push. The unbundle function has to detect which version of the bundle to use since the return type is different. Note that push error handling is currently nonexistent. This is one of the reasons why bundle2 is still disabled by default.