[Dubbo-7054]消费者端Executor改为全局共享，不再按提供者Port共享。

[zhangyz-hd]

What is the purpose of the change

目前社区版本消费者端的executor线程池是按提供者的Port的做了共享，当一个提供者下线时，AbstractClient就会将executor销毁，造成后续处理ChannelState.DISCONNECTED事件，以及消费者再调用其他提供者时无可用的executor，就会重新在创建一个executor，这个模式会导致#7054 所描述的问题。
同时考虑到消费者维护N个executor的开销，因此将消费者端的executor改为全局1个，在节点shutdown时再销毁。

Brief changelog

1，DefaultExecutorRepository分配executor时，消费者的key使用Integer.MAX_VALUE，保证消费者使用1个executor，提供者保持原装（按协议的port分配executor）；同时getExecutor()内不再createExecutor，当executor为null则直接降级返回SHARED_EXECUTOR；
2，AbstractClient在close0时不关闭其持有的executor；
3，DubboBootstrap.destroy()时销毁ExecutorRepository；

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[codecov-io]

Codecov Report

Merging #7109 (7c8dfcd) into master (bd43fa7) will decrease coverage by 0.54%.
The diff coverage is 81.81%.

@@             Coverage Diff              @@
##             master    #7109      +/-   ##
============================================
- Coverage     60.07%   59.53%   -0.55%     
+ Complexity      506      287     -219     
============================================
  Files          1028     1001      -27     
  Lines         41531    39918    -1613     
  Branches       6038     5904     -134     
============================================
- Hits          24951    23765    -1186     
+ Misses        13813    13448     -365     
+ Partials       2767     2705      -62     

Impacted Files	Coverage Δ	Complexity Δ
.../threadpool/manager/DefaultExecutorRepository.java	83.87% <78.94%> (-4.27%)	0.00 <0.00> (ø)
.../apache/dubbo/config/bootstrap/DubboBootstrap.java	41.88% <100.00%> (-0.16%)	0.00 <0.00> (ø)
...pache/dubbo/remoting/transport/AbstractClient.java	65.71% <100.00%> (+2.70%)	0.00 <0.00> (ø)
...che/dubbo/common/utils/CharSequenceComparator.java	0.00% <0.00%> (-100.00%)	0.00% <0.00%> (ø%)
...che/dubbo/rpc/cluster/support/FailfastCluster.java	0.00% <0.00%> (-100.00%)	0.00% <0.00%> (ø%)
...bo/rpc/cluster/interceptor/ClusterInterceptor.java	0.00% <0.00%> (-100.00%)	0.00% <0.00%> (ø%)
...java/org/apache/dubbo/remoting/etcd/Constants.java	0.00% <0.00%> (-100.00%)	0.00% <0.00%> (ø%)
...pache/dubbo/metadata/report/support/Constants.java	0.00% <0.00%> (-100.00%)	0.00% <0.00%> (ø%)
...igurator/override/OverrideConfiguratorFactory.java	0.00% <0.00%> (-100.00%)	0.00% <0.00%> (ø%)
...e/dubbo/registry/status/RegistryStatusChecker.java	0.00% <0.00%> (-81.25%)	0.00% <0.00%> (ø%)
... and 175 more

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[kevinw66]

LGTM

[wongoo]

@zhangyz-hd pls check this issue #8172, do u have any idea to fix it?