Abstract db access, allow for caching middleware

[applejag]

We should abstract out the DB access to an interface.

• Easier to test
• Easier to switch out the DB implementation (maybe we want to move away from GORM?)
• Easier to add middlewares such as a in-memory or Redis based cache
• Easier to extract some parts of the DB to separate data stores, such as storing artifacts in an S3 instead of inside Postgres or logs inside elasticsearch

The database should be a plugin.