Swith to the minimal feature set by default

Summary:
The docker-registry-api uses the following dependency:
limiting-factor = { path="../limiting-factor", features = ["minimal"] }

Yet, a Jenkins task to generate doc compiles it with PostgreSQL support.

It appears the default feature set is always used.

Fixes T1459.

Test Plan: Rebuild this crate, then the API crate.

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1459

Differential Revision: https://devcentral.nasqueron.org/D1886