protected void renderContinuations(Map<String, Continuation> continuations) throws IOException {
        if (continuations.isEmpty()) return;

        generator.writeObjectFieldStart(CONTINUATION);
        for (Map.Entry<String, Continuation> e : continuations.entrySet()) {
            generator.writeStringField(e.getKey(), e.getValue().toString());
        }
        generator.writeEndObject();
    }
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