streamclone: add support for bundle2 based stream clone
The feature put to use the various bits introduced previously. If the server
supports it, the client will request its stream clone through bundle2 instead of
the legacy 'stream_out' commands. The bundle2 version use the better 'v2'
version of stream bundles.
The 'v2' format is not finalized yet. Now that there are some code running it,
we can start working on it again.
Performance numbers are available at the end of this series.