private static String getBlobStoreType(BlobStore blobStore) {
        return blobStore.getContext().unwrap().getProviderMetadata().getId();
    }
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