Allow for other GORM databases

[wishdev]

Adds ability to use PostgreSQL or other GORM drivers for the database

[poundifdef]

Thanks for doing this - it's taken me some time to look! I like the idea of supporting postgres+mysql using gorm. A few high-level comments:

• I think it would be helpful to have the sqlite and postgres implementation actually implement the Queue interface, even if they are just calling Database.CreateQueue(). A couple of reasons for this:
  • The sqlite implementation uses a mutex for all writes. However, in postgres, that is not necessary, since the DB handles that
  • It would be cleaner if only the postgres module depended on gorm/postgres. As written, the database module depends on gorm/postgres but not gorm/sqlite. I think there could be a clearer separation of concerns
  • There may be cases where we want to override the default gorm implementation with custom sql

I would also prefer that the config have distinct "postgres" vs "sqlite" configs, as opposed to a generic "db" with postgres/sqlite subtypes. The reason is that if (when) we add additional database types not supported by gorm, the configuration format will be consistent.

Have other smaller stylistic comments, but that is what I'm thinking at the moment.