Abstract backend storage

[dienummer]

Initial abstraction with working Postgres and stub Rocksdb

TBD:

• abstract startup (db connection intialization)
• decide how to select backend in config

[dienummer]

Shoot I forgot to cargo fmt and to rip out the unused code in VssItem. I'll wait on comments before further commits

[benthecarman]

How much do you need this? I'd rather not have to complicate the logic for this unless postgres is really a non-starter for people, which I don't see why it would be

[dienummer]

How much do you need this? I'd rather not have to complicate the logic for this unless postgres is really a non-starter for people, which I don't see why it would be

Hey, thanks for taking a look. I don't need it, but it would be undoubtedly much more convenient to deploy a single binary or container with SQLite or Rocksdb as a backend.

Additionally, in my opinion this makes the logic less complicated (see put_objects_impl for instance), but reasonable people can disagree on this.

In any case, you may wish to leave this open until the server is more widely used, to get a sense of whether others also feel removing the postgres dependency eases deployment.

Cheers