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.

bundle2: allow bundle2 for pulling over the wire This changeset makes `wireprotocol` peers advertise bundle2 capability and comply with bundle2 `getbundle` requests. Note that advertising bundle2 could make a client try to use it for push. Such pushes would fail. However, I do not expect any human being to have enabled bundle2 on their server yet.

bundle2: return a stream from exchange.getbundle For friendliness with the wire protocol implementation, the `exchange.getbundle` now returns a binary stream when asked for a bundle2. We detect a bundle2 request and upgrade the binary stream to an unbundler object. In the future the unbundler may gain feature to look like a binary stream, but we are not quite there yet.