Owner: Fix gcttl bug (#1861)

[ti-chi-bot]

This is an automated cherry-pick of #1861

What problem does this PR solve?

#1839

What is changed and how it works?

Modify the calculation method of MinGCSafePoint （the lower bound of CDC's gcSafePoint）

Before：
MinGCSafePoint = curentTs - gcTTL

After：
MinGCSafePoint = min(curentTs - gcTTL, pdSafePoint)

Check List

Tests

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

Side effects

• Increased code complexity

Related changes

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

Release note

• owner: When tikv_gc_life_time is greater than gcttl, use tikv_gc_life_time to calculate the lower bound of gcSafePoint.

[asddongmen]

/lgtm

[ti-chi-bot]

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[zier-one]

/merge

[ti-chi-bot]

@leoppro: /merge in this pull request requires 2 approval(s).

In response to this:

/merge

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[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• leoppro
• liuzix

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[asddongmen]

/run-leak-tests

[amyangfei]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 012cb9a

[codecov-commenter]

Codecov Report

Merging #1871 (8c93f79) into release-4.0 (753073a) will decrease coverage by 0.1629%.
The diff coverage is 62.7033%.

@@                 Coverage Diff                 @@
##           release-4.0      #1871        +/-   ##
===================================================
- Coverage      53.7661%   53.6032%   -0.1630%     
===================================================
  Files              153        163        +10     
  Lines            15958      17290      +1332     
===================================================
+ Hits              8580       9268       +688     
- Misses            6475       7042       +567     
- Partials           903        980        +77