[Dubbo-1876] Enhancements for the new async way of Dubbo

[chickenlj]

What is the purpose of the change

Enhancements for the new async way of Dubbo introduced in #1876. Below is the main issues this PR trys to solve:

• Support service signature with original CompletableFuture returntype.

      public interface DemoService {
          CompletableFuture<String> sayHello(String name);
      }

• Provide context restore API when switching threads.

      AsyncContext.signalContextSwitch();

• Postprocess of async result in filter chain.

Brief changelog

• Add AsyncRpcResult to represent result value of async invoke, equivalent to RpcResult for sync invoke.

• Add PostProcessFilter to support postprocess of result in filter chain

• Change AbstractProxyInvoker and DubboInvoker to support services with original Future signature, supplementary for generated Future service.

Verifying this change

• Run samples for how to use async in a new way.
  • dubbo-samples-async-original-future
  • dubbo-samples-async
  • dubbo-samples-async-generated-future
  • dubbo-samples-async-provider
• Still lack of UT, expect to add soon.

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[codecov-io]

Codecov Report

Merging #1957 into master will increase coverage by <.01%.
The diff coverage is 38.03%.

@@             Coverage Diff              @@
##             master    #1957      +/-   ##
============================================
+ Coverage     53.07%   53.07%   +<.01%     
- Complexity     4930     4956      +26     
============================================
  Files           557      559       +2     
  Lines         24802    24892      +90     
  Branches       4427     4431       +4     
============================================
+ Hits          13164    13212      +48     
- Misses         9618     9660      +42     
  Partials       2020     2020

Impacted Files	Coverage Δ	Complexity Δ
...oting/exchange/support/ExchangeHandlerAdapter.java	0% <ø> (ø)	0 <0> (ø)	⬇️
.../src/main/java/org/apache/dubbo/rpc/RpcResult.java	72.72% <ø> (+21.44%)	10 <0> (ø)	⬇️
...c/main/java/org/apache/dubbo/common/Constants.java	88.88% <ø> (ø)	1 <0> (ø)	⬇️
...ache/dubbo/rpc/proxy/InvokerInvocationHandler.java	30.76% <0%> (-4.02%)	3 <0> (ø)
...exchange/support/header/HeaderExchangeHandler.java	30.15% <0%> (ø)	10 <0> (ø)	⬇️
...ava/org/apache/dubbo/rpc/SimpleAsyncRpcResult.java	0% <0%> (ø)	0 <0> (?)
...le/src/main/java/com/alibaba/dubbo/rpc/Result.java	22.22% <0%> (-11.12%)	0 <0> (ø)
...main/java/org/apache/dubbo/rpc/AsyncRpcResult.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...apache/dubbo/rpc/filter/ConsumerContextFilter.java	85.71% <100%> (+1.09%)	3 <2> (+1)	⬆️
.../dubbo/rpc/protocol/dubbo/filter/FutureFilter.java	59.09% <100%> (+2.52%)	16 <6> (+3)	⬆️
... and 22 more

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[nobodyiam]

Looks fantastic! This new async way of Dubbo is quite amazing!

(It may be better to squash the commits into one commit)

[chickenlj]

https://github.com/chickenlj/incubator-dubbo/wiki/%E5%A6%82%E4%BD%95%E5%9F%BA%E4%BA%8EDubbo%E5%AE%9E%E7%8E%B0%E5%85%A8%E5%BC%82%E6%AD%A5%E8%B0%83%E7%94%A8%E9%93%BE

An article introducing the new async way was recorded here, written in Chinese.

[chickenlj]

Thank @hengyunabc for help reviewing, he totally agrees with the new async way showed in the demo.
And he'll dive into the code soon.