Executor getExecutor() {
        return beforeHandoverMode ? Runnable::run : renderingExecutor;
    }
Returns the executor in which to execute a listener.
Before handover this *must* be the calling thread, because listeners are free to modify the dataList.
After handover it can be any thread in the renderer pool.
Note that as some listeners may be set up before handover and executed after, it is possible that some rendering
inadvertently work ends up in async data producing threads in some cases.
container-core/src/main/java/com/yahoo/processing/rendering/AsynchronousSectionedRenderer.java