ddl: Fix zero safepoint make schema gc run repeatly

[JaySon-Huang]

What problem does this PR solve?

Issue Number: close #8356

Problem Summary:

After a new tidb cluster deployed, there is an interval that PD will return gc safepoint with 0

>  tiup ctl:nightly pd -u [10.2.12.81:6540](http://10.2.12.81:6540/) -i
Starting component `ctl`: /DATA/disk1/ra_common/.tiup/components/ctl/v7.6.0-alpha-nightly-20231112/ctl pd -u [10.2.12.81:6540](http://10.2.12.81:6540/) -i
» service-gc-safepoint
{
  "service_gc_safe_points": [
    {
      "service_id": "gc_worker",
      "expired_at": 9223372036854775807,
      "safe_point": 445603590956646400
    }
  ],
  "gc_safe_point": 0 <-- tiflash use this value
}

This will lead to SchemaSyncService::gc running repeatedly. This results in lots of useless logging and getting gc safepoint from pd frequently.

What is changed and how it works?

• If tiflash receives a gc safepoint == 0 from PD, it skips the schema gc task
• Refine some logging for adding/removing schema task for keyspace

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

[JaySon-Huang]

/run-all-tests

[Lloyd-Pottiger]

Others LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JinheLin, Lloyd-Pottiger

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• OWNERS [JinheLin,Lloyd-Pottiger]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-11-13 05:38:39.954357658 +0000 UTC m=+4054717.541467787: ☑️ agreed by Lloyd-Pottiger.
• 2023-11-13 05:53:36.862128013 +0000 UTC m=+4055614.449238158: ☑️ agreed by JinheLin.

[JaySon-Huang]

/run-all-tests

[JaySon-Huang]

/run-unit-test

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #8363.