use equal explicit class to replace anonymous class

[tswstarplanet]

What is the purpose of the change

The anonymous class used in AvailableCluster's constructor has a equal explicit class, we can use the explicit one to replace the anonymous one.

Brief changelog

XXXXX

Verifying this change

XXXXX

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[codecov-io]

Codecov Report

Merging #4027 into master will increase coverage by 1.3%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##             master    #4027     +/-   ##
===========================================
+ Coverage     63.42%   64.73%   +1.3%     
- Complexity      564      600     +36     
===========================================
  Files           755      755             
  Lines         32425    34661   +2236     
  Branches       5145     5968    +823     
===========================================
+ Hits          20567    22438   +1871     
- Misses         9496     9830    +334     
- Partials       2362     2393     +31

Impacted Files	Coverage Δ	Complexity Δ
...he/dubbo/rpc/cluster/support/AvailableCluster.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...onfig/spring/extension/SpringExtensionFactory.java	78.04% <0%> (-7.32%)	0% <0%> (ø)
...mmon/threadpool/support/AbortPolicyWithReport.java	83.33% <0%> (-5.56%)	0% <0%> (ø)
...ng/exchange/support/header/HeartbeatTimerTask.java	73.68% <0%> (-5.27%)	0% <0%> (ø)
...c/protocol/dubbo/ReferenceCountExchangeClient.java	66.66% <0%> (-1.86%)	0% <0%> (ø)
...dubbo/remoting/exchange/support/DefaultFuture.java	73.46% <0%> (-1.37%)	0% <0%> (ø)
...ubbo/rpc/protocol/dubbo/ChannelWrappedInvoker.java	47.22% <0%> (-0.7%)	0% <0%> (ø)
...c/main/java/org/apache/dubbo/common/Constants.java	90.9% <0%> (ø)	0% <0%> (ø)	⬇️
...a/org/apache/dubbo/registry/etcd/EtcdRegistry.java	0% <0%> (ø)	0% <0%> (ø)	⬇️
...n/java/org/apache/dubbo/config/ProviderConfig.java	98.02% <0%> (+0.17%)	0% <0%> (ø)	⬇️
... and 44 more

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[ralf0131]

LGTM.