Praefect data model changes with replication

These changes update the data model for praefect with our new scheme of treating praefect as a standalone gitaly storage (from the perspective of gitlab-rails.) We used this branch for the praefect demo on 8/2/19

This MR also adds logic to enable real-time replication

Also, removing the docker cluster because it's not that useful. Instead, we are going to add praefect as an optional service in gdk: #1831 (closed)