public EmbeddedJettyFactory withThreadPool(ThreadPool threadPool) {
        this.threadPool = threadPool;
        return this;
    }
Sets optional thread pool for jetty server.  This is useful for overriding the default thread pool
behaviour for example io.dropwizard.metrics.jetty9.InstrumentedQueuedThreadPool.

@param threadPool thread pool
@return Builder pattern - returns this instance
src/main/java/spark/embeddedserver/jetty/EmbeddedJettyFactory.java