[Dubbo-4570] Fix RegistryProtocol.DestroyableExporter modifiers order

[AnthonyKot]

What is the purpose of the change

Micro refactoring

Brief changelog

Modifiers order changed

Verifying this change

XXXXX

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[AnthonyKot]

Build failed with error not related to PR

[AnthonyKot]

Last try with Travis

[codecov-io]

Codecov Report

Merging #4583 into master will increase coverage by 0.16%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #4583      +/-   ##
============================================
+ Coverage     63.18%   63.34%   +0.16%     
- Complexity      448      450       +2     
============================================
  Files           770      770              
  Lines         33015    33015              
  Branches       5204     5218      +14     
============================================
+ Hits          20860    20913      +53     
+ Misses         9745     9704      -41     
+ Partials       2410     2398      -12

Impacted Files	Coverage Δ	Complexity Δ
...e/dubbo/registry/integration/RegistryProtocol.java	82.69% <ø> (ø)	0 <0> (ø)	⬇️
...che/dubbo/common/compiler/support/JdkCompiler.java	64.54% <0%> (ø)	0% <0%> (ø)	⬇️
...org/apache/dubbo/registry/redis/RedisRegistry.java	48.15% <0%> (+0.28%)	31% <0%> (ø)	⬇️
.../java/org/apache/dubbo/config/ReferenceConfig.java	59.85% <0%> (+0.36%)	0% <0%> (ø)	⬇️
...rg/apache/dubbo/common/timer/HashedWheelTimer.java	63.1% <0%> (+0.68%)	0% <0%> (ø)	⬇️
...org/apache/dubbo/config/context/ConfigManager.java	66.66% <0%> (+0.79%)	0% <0%> (ø)	⬇️
...dubbo/remoting/exchange/support/DefaultFuture.java	86.45% <0%> (+1.04%)	0% <0%> (ø)	⬇️
...ache/dubbo/common/compiler/support/ClassUtils.java	50.26% <0%> (+1.04%)	0% <0%> (ø)	⬇️
...a/org/apache/dubbo/monitor/dubbo/DubboMonitor.java	89.71% <0%> (+1.86%)	16% <0%> (ø)	⬇️
.../exchange/support/header/HeaderExchangeServer.java	66.98% <0%> (+1.88%)	0% <0%> (ø)	⬇️
... and 16 more

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[beiwei30]

LGTM.