Add tag variables to avoid repeated calls to ExporterChangeableWrapper#unexport method

[BurningCN]

When the dubbo program is destroyed, for example, start from spring's context.stop(). DubboBootstrap#destroy will be called, and the ExporterChangeableWrapper#unexport method will be called twice in the logic.

• The first is in destroyProtocols()->protocol.destroy()->Registry#destroy()->exporter.unexport()->ExporterChangeableWrapper#unexport
• the second place is in unexportServices()->sc.unexport()->exporter.unexport()->DestroyableExporter#unexport->ExporterChangeableWrapper#unexport

So, I add this unexported variable to avoid repeated unexport

[codecov-commenter]

Codecov Report

Merging #7819 (6c19340) into master (e845e16) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #7819      +/-   ##
============================================
+ Coverage     59.28%   59.40%   +0.12%     
- Complexity      529      556      +27     
============================================
  Files          1077     1086       +9     
  Lines         43565    43842     +277     
  Branches       6368     6396      +28     
============================================
+ Hits          25826    26043     +217     
- Misses        14879    14913      +34     
- Partials       2860     2886      +26     

Impacted Files	Coverage Δ	Complexity Δ
...e/dubbo/registry/integration/RegistryProtocol.java	60.49% <100.00%> (-0.26%)	0.00 <0.00> (ø)
.../dubbo/remoting/transport/MultiMessageHandler.java	80.00% <0.00%> (-20.00%)	0.00% <0.00%> (ø%)
...bo/rpc/cluster/support/FailbackClusterInvoker.java	52.45% <0.00%> (-18.04%)	0.00% <0.00%> (ø%)
...in/java/org/apache/dubbo/common/utils/JVMUtil.java	81.13% <0.00%> (-11.33%)	0.00% <0.00%> (ø%)
...va/org/apache/dubbo/rpc/support/ProtocolUtils.java	61.53% <0.00%> (-8.03%)	0.00% <0.00%> (ø%)
.../threadpool/manager/DefaultExecutorRepository.java	80.95% <0.00%> (-4.54%)	0.00% <0.00%> (ø%)
.../rpc/protocol/dubbo/LazyConnectExchangeClient.java	49.45% <0.00%> (-4.40%)	0.00% <0.00%> (ø%)
.../apache/dubbo/remoting/transport/AbstractPeer.java	58.69% <0.00%> (-4.35%)	0.00% <0.00%> (ø%)
...ng/transport/dispatcher/WrappedChannelHandler.java	58.69% <0.00%> (-4.35%)	0.00% <0.00%> (ø%)
...n/java/org/apache/dubbo/common/utils/LRUCache.java	82.22% <0.00%> (-3.99%)	0.00% <0.00%> (ø%)
... and 80 more

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[AlbumenJ]

It would be better to use AtomicBoolean, and use cas to set ture here to prevent concurrency problem

[BurningCN]

Thanks, according to your suggestion, it has been adjusted to AtomicBoolean type