sshpeer: add implementation of _calltwowaystream This implements the call needed by bundle2. The error handling is a bit flaky right now, but bundle2 error handling in general is still flaky anyway. Bundle2 is still disabled by default, I do not expect this to be a problem.

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.

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.

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.

bundle2: prepare readbundle to return more that one type of bundle We first read 4 bytes to get the `HG10` bytes then we read the compression scheme if this is `HG10`. This prepares the code for the arrival of `HG20` handling.

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