etcdutil: update member when unavailable

[lhy1024]

What problem does this PR solve?

Issue Number: Close #6442

What is changed and how does it work?

Before etcd support balancer with only leader, we will use a goroutine to detect timeout and update members.

But we need more tests, so it is only used in mcs.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

3 network partition + 3 leader io hang

10 network partition

Release note

None.

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[lhy1024]

I am not sure whether it is suitable.

[codecov]

Codecov Report

Patch coverage: 92.30% and project coverage change: +0.23 🎉

Comparison is base (ac31f87) 74.61% compared to head (62d99de) 74.84%.

❗ Current head 62d99de differs from pull request most recent head b19da74. Consider uploading reports for the commit b19da74 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6462      +/-   ##
==========================================
+ Coverage   74.61%   74.84%   +0.23%     
==========================================
  Files         417      410       -7     
  Lines       43089    41794    -1295     
==========================================
- Hits        32150    31282     -868     
+ Misses       8090     7800     -290     
+ Partials     2849     2712     -137     

Flag	Coverage Δ
unittests	74.84% <92.30%> (+0.23%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/utils/etcdutil/etcdutil.go	83.51% <92.10%> (+3.90%)	⬆️
pkg/mcs/resource_manager/server/server.go	52.99% <100.00%> (ø)
pkg/mcs/tso/server/server.go	53.81% <100.00%> (-0.94%)	⬇️

... and 165 files with indirect coverage changes

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[lhy1024]

It will be fix by #6577