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:

• JaySon-Huang
• flowbehappy

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[lidezhu]

/rebuild

[CLAassistant]

All committers have signed the CLA.

[lidezhu]

/rebuild

[lidezhu]

/rebuild

[lidezhu]

/run-all-tests

[sre-bot]

lines: 43.3% (49809 out of 115126)
branches: 6.4% (81421 out of 1263896)

[lidezhu]

/run-all-tests

[lidezhu]

/run-all-tests

[lidezhu]

@JaySon-Huang PTAL

[JaySon-Huang]

/merge

[ti-chi-bot]

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

/run-all-tests

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[ti-chi-bot]

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

Commit hash: 9e7102b

[sre-bot]

lines: 43.3% (49975 out of 115355)
branches: 6.5% (81768 out of 1265193)

[sre-bot]

lines: 43.3% (49974 out of 115355)
branches: 6.5% (81762 out of 1265193)