Is your feature request related to a problem? Please describe.
For consistency with the Grafeas architecture, we'd like to move the PostgreSQL storage implementation to its own project, grafeas-pgsql.

Describe the solution you'd like

1. Move pgsqlstore.go to grafeas-pgsql.
2. Re-build the Grafeas server with the PostgreSQL storage.

Describe alternatives you've considered
For now, it's part of the Grafeas project, but going forward we'd like to keep the project scope limited to the API.