Support for configurable ThreadPoolExecutor

[fernandezpablo85]

Right now spark supports partial configuration of it's underlying ThreadPool via:

• threadPool(maxThreads)

and

• threadPool(int maxThreads, int minThreads, int idleTimeoutMillis)

Though convenient, these fall short when more fine tuning is needed, like configuring a blocking queue or a custom ThreadFactory.

I'd be great to have something like threadPool(ThreadPoolExecutor executor) or equivalent.

[toyg]

You can achieve finer-level control by adding this to main() before everything else:

EmbeddedServers.add(
        EmbeddedServers.Identifiers.JETTY,
        new EmbeddedJettyFactory().withThreadPool(yourCustomThreadPool)
);

Note the ThreadPool is actually Jetty's org.eclipse.jetty.util.thread.ThreadPool, rather than anything in JDK.