Support async request() convenience API

[mcqueary]

Support an asynchronous 1:N request call allows the user to supply their own MessageHandler callback to handle responses. Something along the lines of:

request(String subject, byte[] data, long timeout, TimeUnit unit, int maxReplies, MessageHandler cb)
request(String subject, byte[] data, int maxReplies, MessageHandler cb)
request(String subject, byte[] data, long timeout, TimeUnit unit, MessageHandler cb)

Thought will need to be given to managing the life of the subscription, probably using a ScheduledExecutorService to terminate the subscription once the timeout is reached.

Thought should also be given to whether this is just provided as example code.

[pires]

Here's how I do it:

private final ScheduledExecutorService ses = Executors.newSingleThreadScheduledExecutor();

public void request(String subject, long timeoutInMillis, MessageHandler handler) {
    // prepare inbox
    final String inbox = nats.newInbox();
    // subscribe to inbox
    final Subscription subscription = nats.subscribeAsync(inbox, handler);
    // schedule unsubscribe
    ses.schedule((Runnable) () -> subscription.close(), timeoutInMillis, TimeUnit.MILLISECONDS);
    // send request
    nats.publish(subject, inbox, null);
}

[ColinSullivan1]

At this time, functionality along these lines should occur outside of the core NATS library.