Add deleteContainerIfEmpty to BlobStore

This matches how most blobstores operate: delete container is a single
operation, not a compound operation which recursively deletes blobs.
I don't know if my comment got lost, but `deleteContainerIfEmptyWithoutContents` exercises this via:

```
assertTrue(view.getBlobStore().deleteContainerIfEmpty(containerName));
assertNotExists(containerName);
assertTrue(view.getBlobStore().deleteContainerIfEmpty(containerName));
```