WIP: Blobstore scheduled garbage collection

Facility for creating a scheduled task to compact a blob store (meaning, deleting marked-for-deletion blobs). Right now, though, the only blob store feature is m2settings, which doesn't need this.  (Settings blobs are so few that it's fine to compact after every removal.)

This PR is just a way to hang on to these changes until such time as we actually need them, so we don't have unused code sitting in master.