refactor: make seata-discovery more readable

[koonchen]

Ⅰ. Describe what this PR did

I found some optimization points when i was reading source code in seata-discovery module.

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov-io]

Codecov Report

Merging #2209 into develop will increase coverage by <.01%.
The diff coverage is 45.45%.

@@              Coverage Diff              @@
##             develop    #2209      +/-   ##
=============================================
+ Coverage      54.32%   54.33%   +<.01%     
  Complexity      2491     2491              
=============================================
  Files            454      454              
  Lines          14880    14872       -8     
  Branches        1753     1749       -4     
=============================================
- Hits            8084     8081       -3     
+ Misses          6025     6020       -5     
  Partials         771      771

Impacted Files	Coverage Δ	Complexity Δ
...a/io/seata/discovery/registry/RegistryFactory.java	0% <ø> (ø)	0 <0> (ø)	⬇️
...ery/registry/consul/ConsulRegistryServiceImpl.java	1.01% <0%> (+0.02%)	1 <0> (ø)	⬇️
...covery/registry/etcd3/EtcdRegistryServiceImpl.java	11.62% <0%> (+0.17%)	3 <0> (ø)	⬇️
...eata/discovery/loadbalance/LoadBalanceFactory.java	50% <100%> (-16.67%)	1 <1> (ø)
...very/registry/zk/ZookeeperRegisterServiceImpl.java	62.29% <100%> (-0.61%)	24 <3> (ø)

[zjinlei]

LGTM

[slievrly]

LGTM

[funky-eyes]

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