Async enhancement

[chickenlj]

What is the purpose of the change

• User does not have to set async=true on Provider side to use provider async feature anymore, only AsyncContext.startAsync() is necessary.
• stop tag from being transferred in the RPC chain

Brief changelog

• AsyncContext related classes
• ContextFilter

Verifying this change

run samples-async

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[codecov-io]

Codecov Report

Merging #3184 into 2.7.0-release will decrease coverage by 0.02%.
The diff coverage is 70%.

@@                Coverage Diff                 @@
##             2.7.0-release   #3184      +/-   ##
==================================================
- Coverage            63.83%   63.8%   -0.03%     
  Complexity              75      75              
==================================================
  Files                  651     651              
  Lines                28317   28311       -6     
  Branches              4793    4792       -1     
==================================================
- Hits                 18076   18064      -12     
- Misses                7973    7975       +2     
- Partials              2268    2272       +4

Impacted Files	Coverage Δ	Complexity Δ
...apache/dubbo/rpc/protocol/dubbo/DubboProtocol.java	66.1% <ø> (-0.57%)	0 <0> (ø)
...g/apache/dubbo/rpc/proxy/AbstractProxyInvoker.java	50% <0%> (ø)	0 <0> (ø)	⬇️
...ava/org/apache/dubbo/rpc/filter/ContextFilter.java	93.1% <100%> (+0.51%)	0 <0> (ø)	⬇️
...in/java/org/apache/dubbo/rpc/AsyncContextImpl.java	58.33% <50%> (+4.48%)	0 <0> (ø)	⬇️
...src/main/java/org/apache/dubbo/rpc/RpcContext.java	71.02% <75%> (+0.23%)	0 <0> (ø)	⬇️
...bbo/registry/support/ProviderConsumerRegTable.java	80.48% <0%> (-4.88%)	0% <0%> (ø)
...onfig/spring/extension/SpringExtensionFactory.java	80.48% <0%> (-4.88%)	0% <0%> (ø)
...e/dubbo/remoting/transport/netty/NettyChannel.java	57.64% <0%> (-4.71%)	0% <0%> (ø)
...he/dubbo/remoting/transport/netty/NettyServer.java	67.85% <0%> (-3.58%)	0% <0%> (ø)
...rpc/protocol/dubbo/telnet/InvokeTelnetHandler.java	72.5% <0%> (-1.67%)	0% <0%> (ø)
... and 5 more

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[beiwei30]

one more thing, should we override setAsync(boolean) on ProviderConfig to warn user this only works on client side?

[chickenlj]

one more thing, should we override setAsync(boolean) on ProviderConfig to warn user this only works on client side?

I will check if setAsync(boolean) is newly added in 2.7.0 or not. If it is newly added, I think we should remove or override it.

[beiwei30]

LGTM

[zhangyl]

why stop tag from being transferred in the RPC chain?