[WIP] use phantom types to restrict roles

[jpfuentes2]

Just an idea I'm tossing around right now. Use phantom types to restrict a Database to specific roles: Primary and Replica. The former can do all-the-things and the latter can do read-only queries.

Learn more about phantom types:

• http://ktoso.github.io/scala-types-of-types/#phantom-type
• https://blog.codecentric.de/en/2016/02/phantom-types-scala/
• http://danielwestheide.com/blog/2015/06/28/put-your-writes-where-your-master-is-compile-time-restriction-of-slick-effect-types.html