Postgres Config Backend schema

The PG CB schema has been developed and reviewed in #90 (closed), but back then was decided to merge it to integration branch feature-pg-cb, because it was unlikely it would be completed in time for 2.0 release. Now with 2.1 dev cycle started, we can merge it. And that's what this ticket is about.

The branch has been rebased recently, so it should be an easy thing.