Adds an in-memory and a Postgres implemenation of repository
state tracking stores. The two tables added are:

1. `repositories` which contains the expected state of repositories
    in the virtual storage.
2. `storage_repositories` which contains the state of the repositories
    on the physical storages.

Cross-referencing these two makes it easier to identify outdated
repositories.

This commit implements only the stores without hooking them up to the
rest of the code.