optimize : use constants as callers to the equals() method

[ggezzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz]

What is the purpose of the change

optimize : use constants as callers to the equals() method

Brief changelog

1.dubbo-cluster/src/main/java/org/apache/dubbo/rpc/cluster/loadbalance/AbstractLoadBalance.java
2.dubbo-common/src/main/java/org/apache/dubbo/common/constants/RegistryConstants.java

Verifying this change

XXXXX

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[codecov-io]

Codecov Report

Merging #5352 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #5352      +/-   ##
============================================
+ Coverage     61.17%   61.19%   +0.02%     
  Complexity      424      424              
============================================
  Files           919      919              
  Lines         37487    37486       -1     
  Branches       5461     5461              
============================================
+ Hits          22933    22940       +7     
+ Misses        12049    12041       -8     
  Partials       2505     2505

Impacted Files	Coverage Δ	Complexity Δ
...o/rpc/cluster/loadbalance/AbstractLoadBalance.java	68.18% <100%> (-1.39%)	0 <0> (ø)
...pache/dubbo/remoting/transport/AbstractServer.java	53.75% <0%> (-3.75%)	0% <0%> (ø)
...g/apache/dubbo/rpc/protocol/rest/RestProtocol.java	68.21% <0%> (-3.11%)	0% <0%> (ø)
...he/dubbo/registry/multicast/MulticastRegistry.java	67.87% <0%> (-1.81%)	0% <0%> (ø)
...a/org/apache/dubbo/monitor/dubbo/DubboMonitor.java	89.71% <0%> (-0.94%)	16% <0%> (-1%)
...g/apache/dubbo/registry/consul/ConsulRegistry.java	62.5% <0%> (+0.62%)	29% <0%> (ø)	⬇️
.../exchange/support/header/HeaderExchangeServer.java	67.92% <0%> (+0.94%)	0% <0%> (ø)	⬇️
...exchange/support/header/HeaderExchangeHandler.java	64.6% <0%> (+2.65%)	0% <0%> (ø)	⬇️
...e/dubbo/remoting/transport/netty/NettyChannel.java	55.68% <0%> (+3.4%)	20% <0%> (+1%)	⬆️
.../org/apache/dubbo/remoting/ExecutionException.java	21.05% <0%> (+5.26%)	0% <0%> (ø)	⬇️
... and 4 more

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[maoyiz]

LGTM

[chickenlj]

LGTM>