WIP Rely on Omnibus hg-inner-http service

Relies on work for omnibus-heptapod#10 (closed) (not ready yet)

This means we can get rid of our wrapper and various circumventing things.

What we lose is the ability to run our migration steps, but we could rehook that one elsewhere if needed (probably we'll want to adopt the mechanism in some other component before the eventual decommission of heptapod-docker)