Internal Mercurial HTTP service

Known as hgserve in the current Docker image, it would be much more consistent to manage it directly with Omnibus.

This is a good opportunity to give it a better name, taking into account that it'll be alongside hgitaly, same for the logs directory name.

The service should probably called hg-inner-http, to tell readily that it's for HTTP only, and not directly exposed (that's the job of Heptapod Workhorse).

Those are mild breaking changes for people managing installations, and a new Heptapod series is a good occasion to do that.

We can also probably take that opportunity to make various Gunicorn params (workers…) modifiable from the standard config instead of the ad-hoc environment variable.