server: Auto fix gc_worker's service safepoint for upgraded clusters (#3371)

[ti-srebot]

cherry-pick #3371 to release-5.0-rc
You can switch your code base to this Pull Request by using git-extras:

# In pd repo:
git pr https://github.com/tikv/pd/pull/3392

After apply modifications, you can push your change to this PR via:

git push [email protected]:ti-srebot/pd.git pr/3392:release-5.0-rc-cbb89ec08597

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Signed-off-by: MyonKeminta [email protected]

What problem does this PR solve?

Fixes #3366

Though we did a fix in PR #3146 , there's still a problem for clusters upgraded from older version, where the gc_worker's service safepoint may be invalid or missing and there are other service safepoints in the cluster.

What is changed and how it works?

Checks if "gc_worker"'s service safepoint exist and has a infinite TTL every time loading safepoints, and tries to fix it if possible.

Check List

Tests

• Unit test

Code changes

Side effects

• Increased code complexity

Related changes

• Need to cherry-pick to the release branch
  • release-4.0

Release note

• No release note

[ti-chi-bot]

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[ti-srebot]

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[codecov]

Codecov Report

Merging #3392 (e536770) into release-5.0-rc (f4e7924) will decrease coverage by 0.04%.
The diff coverage is 68.75%.

@@                Coverage Diff                 @@
##           release-5.0-rc    #3392      +/-   ##
==================================================
- Coverage           74.12%   74.07%   -0.05%     
==================================================
  Files                 243      243              
  Lines               23088    23101      +13     
==================================================
- Hits                17113    17112       -1     
- Misses               4401     4411      +10     
- Partials             1574     1578       +4     

Flag	Coverage Δ
unittests	74.07% <68.75%> (-0.05%)	⬇️

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

Impacted Files	Coverage Δ
server/core/storage.go	67.68% <66.66%> (-0.32%)	⬇️
server/grpc_service.go	55.99% <100.00%> (-0.69%)	⬇️
pkg/errs/errs.go	75.00% <0.00%> (-25.00%)	⬇️
server/election/lease.go	89.83% <0.00%> (-3.39%)	⬇️
server/member/member.go	65.21% <0.00%> (-3.27%)	⬇️
server/tso/allocator_manager.go	75.00% <0.00%> (-0.81%)	⬇️
server/tso/tso.go	78.61% <0.00%> (-0.63%)	⬇️
server/server.go	73.25% <0.00%> (-0.48%)	⬇️
server/election/leadership.go	81.70% <0.00%> (ø)
... and 4 more

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