Add deleteContainerIfEmpty to BlobStore

This matches how most blobstores operate: delete container is a single
operation, not a compound operation which recursively deletes blobs.
> Added suggested test to deleteContainerIfEmpty where it belongs.

Perhaps I'm missing something here, but wouldn't this be something like `deleteContainerIfEmptyReturnsTrueIfContainerNotFound`, rather than just a true/false check if the container exists and is empty/not empty?

/cc @shrinandj