client/tso: fix the issue where the TSO follower proxy cannot be closed (#8719)

[ti-chi-bot]

This is an automated cherry-pick of #8719

What problem does this PR solve?

Issue Number: close #8709.

What is changed and how does it work?

Remove outdated follower connections after disabling the TSO follower proxy.

Check List

Tests

• Unit test
• Integration test

Release note

Fix the issue where the TSO follower proxy cannot be closed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.75%. Comparing base (697d907) to head (5e88582).
Report is 1 commits behind head on release-8.4.

Additional details and impacted files

@@               Coverage Diff               @@
##           release-8.4    #8721      +/-   ##
===============================================
+ Coverage        71.72%   71.75%   +0.03%     
===============================================
  Files              517      517              
  Lines            67450    67452       +2     
===============================================
+ Hits             48377    48400      +23     
+ Misses           15504    15493      -11     
+ Partials          3569     3559      -10     

Flag	Coverage Δ
unittests	71.75% <100.00%> (+0.03%)	⬆️

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-10-16 06:52:46.112072373 +0000 UTC m=+426763.260982203: ☑️ agreed by JmPotato.
• 2024-10-16 07:16:14.103309062 +0000 UTC m=+428171.252218888: ☑️ agreed by okJiang.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JmPotato, okJiang, rleungx

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