Added TiKV RPC CLI and flush log backup tasks in graceful shutdown (#6057)

[ti-chi-bot]

This is an automated cherry-pick of #6057

What problem does this PR solve?

During rolling restart, the lag of log backup may be increased because progress lost of the restarted TiKV.

Closes #6046

What is changed and how does it work?

This added a gRPC Client to TiKV and manually triggered FlushNow when gracefully restarting a TiKV.

Code changes

• Has Go code change
• Has CI related scripts change

Tests

• Unit test
• E2E test
• Manual test
• No code

Side effects

• Breaking backward compatibility
• Other side effects:

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release Notes

Please refer to Release Notes Language Style Guide before writing the release note.

[ti-chi-bot]

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 23.36449% with 82 lines in your changes missing coverage. Please review.

Project coverage is 57.00%. Comparing base (c23a794) to head (18f9050).
Report is 21 commits behind head on release-1.6.

Additional details and impacted files

@@               Coverage Diff               @@
##           release-1.6    #6101      +/-   ##
===============================================
- Coverage        57.20%   57.00%   -0.20%     
===============================================
  Files              259      260       +1     
  Lines            33233    45149   +11916     
===============================================
+ Hits             19010    25739    +6729     
- Misses           12290    17420    +5130     
- Partials          1933     1990      +57     

Flag	Coverage Δ
unittest	57.00% <23.36%> (-0.20%)	⬇️