public static DocumentDeserializer createHead(DocumentTypeManager manager, GrowableByteBuffer buf) {
        return new VespaDocumentDeserializerHead(manager, buf);
    }
Creates a de-serializer for the current head document format.
This format is an extension of the 6.x format.
document/src/main/java/com/yahoo/document/serialization/DocumentDeserializerFactory.java