void renderDataListStart() throws IOException {
            if ( ! listStartIsRendered) {
                if (list instanceof ParentOfTopLevel)
                    beginResponse(stream);
                else
                    beginList(list);
                listStartIsRendered = true;
            }
        }
Resumes rendering data from the current position.
Called both on completion (by this), and when new data is available (from the new data listener).

@return whether this started rendering
container-core/src/main/java/com/yahoo/processing/rendering/AsynchronousSectionedRenderer.java