Implement hgitaly server command
We need a startup server command
- it should follow the same design principles as GitLab (one single HTTP/2 connection etc)
- it should be wrapped as a Mercurial extension: that's what the hg world expects and it gives us configuration, help page etc, something like:
hg hgitaly-server <repositories-root> <bind-address>
I'm not totally settled on the name because I suppose we will eventually need to diverge from the Gitaly proto (be it only to add stuff for Mercurial-only concepts such as phases and obsolescence).
Opinions welcome, of course.