This folder contains only the redis database because the rest (two so far, 'file' and 'leveldb') were created by the Community.
So go there and find more about them. Database is just an
interface so you're able to UseDatabase(anyCompatibleDatabase). A Database should implement two functions, Load and Update.
Database interface
type Database interface {
Load(string) map[string]interface{}
Update(string, map[string]interface{})
}