private void childrenArray() throws IOException {
        if (renderedChildren.peek() == 0)
            generator.writeArrayFieldStart(CHILDREN);
        renderedChildren.push(renderedChildren.pop() + 1);
    }
Creates a json renderer using a custom executor.
Using a custom executor is useful for tests to avoid creating new threads for each renderer registry.
container-search/src/main/java/com/yahoo/search/rendering/JsonRenderer.java