*: add service gc safepoint commands (#2797)

[ti-srebot]

cherry-pick #2797 to release-4.0

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What problem does this PR solve?

Add service gc safepoint commands. Resolve #2786

Tests

• Unit test

Code changes

• Has HTTP API interfaces change (Don't forget to add the declarative for API)

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

Release note

• Add service gc safepoit commands

[ti-srebot]

@overvenus please accept the invitation then you can push to the cherry-pick pull requests.
https://github.com/ti-srebot/pd/invitations

[overvenus]

/run-unit-test

[codecov]

Codecov Report

Merging #2813 into release-4.0 will increase coverage by 0.17%.
The diff coverage is 79.09%.

@@               Coverage Diff               @@
##           release-4.0    #2813      +/-   ##
===============================================
+ Coverage        77.16%   77.34%   +0.17%     
===============================================
  Files              204      208       +4     
  Lines            21940    22765     +825     
===============================================
+ Hits             16931    17608     +677     
- Misses            3724     3827     +103     
- Partials          1285     1330      +45     

Impacted Files	Coverage Δ
client/base_client.go	90.60% <ø> (ø)
client/metrics.go	100.00% <ø> (ø)
pkg/apiutil/apiutil.go	67.34% <ø> (ø)
pkg/apiutil/serverapi/middleware.go	73.07% <ø> (ø)
pkg/cache/cache.go	0.00% <ø> (ø)
pkg/cache/fifo.go	100.00% <ø> (ø)
pkg/cache/lru.go	80.35% <ø> (ø)
pkg/cache/two_queue.go	66.66% <ø> (ø)
pkg/codec/codec.go	78.57% <ø> (ø)
pkg/component/manager.go	81.01% <ø> (ø)
... and 206 more

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[rleungx]

LGTM

[disksing]

/merge

[ti-srebot]

/run-all-tests

[ti-srebot]

@ti-srebot merge failed.

[disksing]

/merge

[ti-srebot]

/run-all-tests