Fix race condition (also a regression of the PR 19139)

[ahrtr]

Fix #19172, which is a regression caused by #19139

I decided to partially rollback 19139 to fix #19172. We need to backport the fix to 3.5, so let's keep it as simple as possible.

We can do any further enhance in 3.6 only.

cc @fuweid @ivanvc @jmhbnz @serathius

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@ahrtr: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-etcd-integration-2-cpu-arm64	7e72e74	link	true	/test pull-etcd-integration-2-cpu-arm64
pull-etcd-integration-4-cpu-arm64	7e72e74	link	true	/test pull-etcd-integration-4-cpu-arm64
pull-etcd-integration-1-cpu-arm64	7e72e74	link	true	/test pull-etcd-integration-1-cpu-arm64
pull-etcd-integration-2-cpu-amd64	7e72e74	link	true	/test pull-etcd-integration-2-cpu-amd64
pull-etcd-integration-4-cpu-amd64	7e72e74	link	true	/test pull-etcd-integration-4-cpu-amd64
pull-etcd-integration-1-cpu-amd64	7e72e74	link	true	/test pull-etcd-integration-1-cpu-amd64

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.79%. Comparing base (88b3938) to head (7e72e74).

Additional details and impacted files

Files with missing lines	Coverage Δ
server/embed/etcd.go	75.73% <ø> (-0.13%)	⬇️

... and 14 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19231      +/-   ##
==========================================
- Coverage   68.87%   68.79%   -0.08%     
==========================================
  Files         420      420              
  Lines       35642    35639       -3     
==========================================
- Hits        24549    24519      -30     
- Misses       9675     9698      +23     
- Partials     1418     1422       +4     

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[ahrtr]

We can't remove the waitgroup otherwise it may cause another race condition. So we have to follow #19221