There was this weird bug happening only when `SQLLAB_BACKEND_PERSISTENCE
= False` where if no database had been selected, there would be some
issue.
This makes sure that the store is populated consistently with the list
of database connections regardless of `SQLLAB_BACKEND_PERSISTENCE`. It
also uses the DAO to fetch database and will apply the RBAC-related
filters if any, the same way that the API does.