[DUBBO-3137]: step3 - start to use RpcConstants

[CrazyHZM]

What is the purpose of the change

issue #3137:
step3 - start to use RpcConstants

Brief changelog

XXXXX

Verifying this change

XXXXX

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[beiwei30]

LGTM.

[beiwei30]

but CI fails due to this:

[ERROR] src/main/java/org/apache/dubbo/rpc/RpcContext.java:[19,8] (imports) UnusedImports: Unused import - org.apache.dubbo.common.Constants.
[ERROR] src/main/java/org/apache/dubbo/rpc/protocol/ProtocolListenerWrapper.java:[19,8] (imports) UnusedImports: Unused import - org.apache.dubbo.common.Constants.
[ERROR] src/main/java/org/apache/dubbo/rpc/protocol/ProtocolFilterWrapper.java:[19,8] (imports) UnusedImports: Unused import - org.apache.dubbo.common.Constants.

[codecov-io]

Codecov Report

Merging #4038 into master will decrease coverage by 0.37%.
The diff coverage is 69.84%.

@@             Coverage Diff              @@
##             master    #4038      +/-   ##
============================================
- Coverage     63.46%   63.08%   -0.38%     
  Complexity      565      565              
============================================
  Files           753      753              
  Lines         32416    32411       -5     
  Branches       5143     5143              
============================================
- Hits          20572    20448     -124     
- Misses         9484     9615     +131     
+ Partials       2360     2348      -12

Impacted Files	Coverage Δ	Complexity Δ
...apache/dubbo/rpc/cluster/router/tag/TagRouter.java	35.86% <ø> (ø)	0 <0> (ø)	⬇️
...pache/dubbo/monitor/dubbo/DubboMonitorFactory.java	100% <ø> (ø)	6 <0> (ø)	⬇️
...va/org/apache/dubbo/rpc/filter/TpsLimitFilter.java	100% <ø> (ø)	0 <0> (ø)	⬇️
...g/apache/dubbo/config/AbstractReferenceConfig.java	94% <ø> (ø)	0 <0> (ø)	⬇️
...c/main/java/org/apache/dubbo/common/Constants.java	75% <ø> (-8.34%)	0 <0> (ø)
...e/dubbo/registry/integration/RegistryProtocol.java	82.69% <ø> (ø)	0 <0> (ø)	⬇️
...ache/dubbo/remoting/transport/mina/MinaServer.java	63.33% <ø> (ø)	6 <0> (ø)	⬇️
...src/main/java/org/apache/dubbo/rpc/RpcContext.java	70% <0%> (ø)	0 <0> (ø)	⬇️
...n/java/org/apache/dubbo/rpc/filter/EchoFilter.java	75% <0%> (ø)	0 <0> (ø)	⬇️
.../main/java/org/apache/dubbo/rpc/RpcInvocation.java	63.95% <0%> (ø)	0 <0> (ø)	⬇️
... and 61 more

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