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.

bundle2: add a "check:heads" handler This part is intended to hold the same role as the `heads` argument of the unbundle function. The client fill it with the known heads at bundle time and the server will abort if its heads changed.