Fixes for the new importing logic:

* Blobs now track the BlobImports and KeyCreators working on them, much like
  Commits track the CommitImporters/BlobImporters working on them. Blobs now
  have a loading column that works identically to Commit.
* Upgraded blobs_keys to a full-blown model, a la CommitsKey, and used more
  straightforward ActiveRecord to do creation and deletion of these records,
  in order to avoid duplicate key errors.
* Fixed a bug where blobs_keys entries would be incorrectly deleted.
* Fixed a bug or two regarding the correctness of the Commit worker list.