What is the purpose of the change

Async support for provider and consumer.

• Consumer, support method with extra Future return type

@AsyncFor(GreetingsServices.class)
public interface GreetingsServiceAsync extends GreetingsServices {
    CompletableFuture<String> sayHi(String name);
}

We also provide a tool to do that: dubbo-async-processor

• Provider, support async like what Servlet 3.0 async works.

public class AsyncServiceImpl implements AsyncService {

    public String sayHello(String name) {
        System.out.println("Main sayHello() method start.");
        final AsyncContext asyncContext = RpcContext.startAsync();
        new Thread(() -> {
            System.out.println("    -- Async start.");
            try {
                Thread.sleep(500);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            asyncContext.write("Hello " + name + ", response from provider.");
            System.out.println("    -- Async end.");
        }).start();
        System.out.println("Main sayHello() method end.");
        return "hello, " + name;
    }
}

Drawbacks and following enhancements:

• For both consumer and provider async, response can't be processed with Filter chains anymore.
• Context still can't be restored between threads.

Thanks for @yuyijq for contributing code and ideas with #606.

Brief changelog

Verifying this change

This change has been deploy to central maven repository with a 2.6.3-SNAPSHOT version. You can see details of how to use from these two projects:
dubbo-async-processor
dubbo-samples

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