Gitaly/Praefect: Protect users from duplicate storage name definitions

Summary

It is possible for users to define external Gitaly and/or Praefect storage targets that share the same name. We should implement a check-config which has the ability to protect against this.

Found in !1757 (comment 508255905), but not specific to that MR.

Configuration Used

The following will result in some seriously odd errors, and will be hard to debug for those unfamiliar with the culprit:

global:
  gitaly:
    internal:
      names:
        - nearline-storage
    external:
      - name: default
        hostname: gitaly-default.infra.internal
        port: 8075
  praefect:
    enabled: true