fix empty segment cannot merge after gc

[lidezhu]

This is an cherry-pick of #4500

What problem does this PR solve?

Issue Number: close #4511

Problem Summary:

1. empty segments cannot be merged after gc;
2. when a segment can be triggerred to gc because of too many delete range, we still wait for the gc_safe_point change, which is not needed.

What is changed and how it works?

1. when segment should be gced because of too many delete range, do it regardless of the gc_safe_point;
2. only update index cache for read thread;
3. fix unstable rough set filter test because of more fast gc speed.

Check List

Tests

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

1. load data into tiflash and check segment status;
   
2. set tiflash replica to 0 and check segment status after a while
   

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

Fix the problem that empty segments cannot be merged after gc

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• JinheLin
• flowbehappy

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[CLAassistant]

All committers have signed the CLA.

[lidezhu]

/run-all-tests

[lidezhu]

/run-all-tests

[lidezhu]

/run-all-tests

[lidezhu]

/run-all-tests

[sre-bot]

lines: 36.0% (42983 out of 119434)
branches: 5.4% (65887 out of 1221517)

[lidezhu]

/merge

[ti-chi-bot]

@lidezhu: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

You only need to trigger /merge once, and if the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

If you have any questions about the PR merge process, please refer to pr process.

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[ti-chi-bot]

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

Commit hash: 9e77ccc

[JaySon-Huang]

/rebuild

[ti-chi-bot]

@lidezhu: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.

[sre-bot]

lines: 35.9% (42783 out of 119299)
branches: 5.4% (65593 out of 1221206)