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.

bundle2: let readbundle return unbundle20 The `readbundle` function can now recognize a bundle2 stream and return the appropriate unbundler. This is required for proper bundle2 support over the wire.

bundle2: make header reading optional The `readbundle` function will consume the 4 first bytes to dispatch between various unbundler. We introduce a way to inform `unbundle20` that the header has been read and it can be trusted.