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.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).