Writes blob references for a given set of hashes.

def WriteHashBlobReferences(self, references_by_hash, cursor):
    """Writes blob references for a given set of hashes."""
    values = []
    for hash_id, blob_refs in iteritems(references_by_hash):
      refs = rdf_objects.BlobReferences(items=blob_refs).SerializeToString()
      values.append({
          "hash_id": hash_id.AsBytes(),
          "blob_references": refs,
      })
    _Insert(cursor, "hash_blob_references", values)