wireproto: add a _calltwowaystream method on wirepeer This method will be used by bundle2 pushes. It calls a command, feeds it with a stream and receives another stream in reply. Actual implementation for ssh and http will be done in later changesets.

bundle2: return a bundle20 object from exchanges.unbundle When a bundle2 is pushed we return a bundle instead of an integer. We use to return a binary stream. We now return a `bundle20` bundler to make the life of wireprotocol implementation simpler.

bundle2: feed a binary stream to `peer.unbundle` This input will have to travel over the wire anyway, so we feed the peer method with a simple binary stream and rely on the server side to use `readbundle` to create the python object. The test output changes because the bundle is created marginally sooner and the debug output interleaves in a different way.