fix empty segment cannot merge after gc and avoid write index data for empty dmfile (#4500)

[ti-chi-bot]

This is an automated 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. the stable size is not 0 for empty dmfile because there is a '0' in every index file in such case;
3. 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

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[ti-chi-bot]

@ti-chi-bot: This cherry pick PR is for a release branch and has not yet been approved by release team.
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[lidezhu]

I will resolve the conflict in this pr after this pr: #3695 merged.

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[lidezhu]

This is not a critical problem and won't merge into release-5.2.