Inner Mercurial HTTP service

This MR provides #10 (closed).

It cannot be tested directly in the current Docker images, because

1. we don't have a heptapod/partial image that includes this MR.
2. the new hg-inner-http service would compete for port with the existing hgserve service.

To test it one needs to

1. build the Docker image with heptapod-docker!19 (closed)

2. mount the Chef cook books directly:

   docker run --detach --hostname heptapod-omnibus --publish 81:80 --publish 2022:22 \
         --name heptapod-omnibus \
         --volume $HOME/heptapod-volumes/omnibus/var:/var/opt/gitlab \
         --volume $HOME/heptapod/omnibus-heptapod/files/gitlab-cookbooks/gitlab:/opt/gitlab/embedded/cookbooks/gitlab \
         octobus/heptapod:testing-py3

I've been able to run the functional tests successfully this way (to be honest, skipping two unrelated ones because of hostname inconsistency)