Add deleteContainerIfEmpty to BlobStore

This matches how most blobstores operate: delete container is a single
operation, not a compound operation which recursively deletes blobs.
> but deleteContainerIfEmptyWithoutContents exercises this via:

Ah, doh, yes...sorry. Perhaps add a `// verify that false is returned even if the container does not exist` before the last assert or as a comment above the test?