[release-3.6] minor refactor DowngradeUpgradeMembersByID to not block on waiting fo…

[ahrtr]

…r member to be ready

backport to 3.6

cc @fuweid

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

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This pull-request has been approved by: ahrtr, fuweid

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.88%. Comparing base (abaa1f0) to head (4a842d1).
Report is 2 commits behind head on release-3.6.

Additional details and impacted files

see 28 files with indirect coverage changes

@@             Coverage Diff              @@
##           release-3.6   #19638   +/-   ##
============================================
  Coverage        68.87%   68.88%           
============================================
  Files              420      420           
  Lines            35787    35787           
============================================
+ Hits             24650    24653    +3     
- Misses            9706     9710    +4     
+ Partials          1431     1424    -7     

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