bugfix: thread safety issue when adding and removing instances

[funky-eyes]

• I have registered the PR changes.

Ⅰ. Describe what this PR did

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]

Codecov Report

Attention: Patch coverage is 78.72340% with 10 lines in your changes missing coverage. Please review.

Project coverage is 50.75%. Comparing base (9d74553) to head (0d0ed41).
Report is 1 commits behind head on 2.x.

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x    #6668      +/-   ##
============================================
+ Coverage     50.72%   50.75%   +0.03%     
- Complexity     5703     5711       +8     
============================================
  Files          1034     1035       +1     
  Lines         35854    35870      +16     
  Branches       4255     4260       +5     
============================================
+ Hits          18188    18207      +19     
+ Misses        15846    15838       -8     
- Partials       1820     1825       +5     

Files	Coverage Δ
...eata/namingserver/entity/vo/monitor/ClusterVO.java	0.00% <ø> (ø)
...eata/namingserver/entity/vo/monitor/WatcherVO.java	0.00% <ø> (ø)
...ache/seata/server/controller/HealthController.java	50.00% <ø> (ø)
...pache/seata/common/metadata/namingserver/Unit.java	0.00% <0.00%> (ø)
...eata/namingserver/controller/HealthController.java	50.00% <50.00%> (ø)
...eata/namingserver/controller/NamingController.java	38.88% <75.00%> (+1.05%)	⬆️
...he/seata/namingserver/entity/pojo/ClusterData.java	63.79% <85.71%> (ø)
...ache/seata/namingserver/manager/NamingManager.java	59.13% <80.76%> (-0.08%)	⬇️

... and 12 files with indirect coverage changes

[xingfudeshi]

LGTM