@Override
                public Map.Entry<String, BlobStore> locateBlobStore(
                        String identityArg, String container, String blob) {
                    return anonymousBlobStore;
                }
S3 supports arbitrary keys for the marker while some blobstores only
support opaque markers.  Emulate the common case for these by mapping
the last key from a listing to the corresponding previously returned
marker.
src/main/java/org/gaul/s3proxy/S3ProxyHandler.java