@Nullable
                @Override
                public Map.Entry<String, BlobStore> locateBlobStore(
                        String identityArg, String container, String blob) {
                    if (!identity.equals(identityArg)) {
                        return null;
                    }
                    return Map.entry(credential, blobStore);
                }
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