fill size in SSTMeta (#233)

[ti-srebot]

cherry-pick #233 to release-4.0
You can switch your code base to this Pull Request by using git-extras:

# In br repo:
git pr https://github.com/pingcap/br/pull/702

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/br.git pr/702:release-4.0-634701b9f73e

---

What problem does this PR solve?

Fix #163

What is changed and how it works?

fill size in sstmeta. but we need make sure this change work on tikv which has this
tikv/tikv#7256, otherwise it will panic tikv.

Check List

Tests

• Unit test
• Integration test

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch

Release note

• Fix the issue that missing fileSize in SSTMeta might cause TiKV to generate a big region.

[sre-bot]

No release note, Please follow https://github.com/pingcap/community/blob/master/contributors/release-note-checker.md

[3pointer]

/run-all-tests

[3pointer]

/rebuild

[overvenus]

Please add a version check, otherwise it may cause TiKV panic.

[3pointer]

LGTM

[3pointer]

Please add a version check, otherwise it may cause TiKV panic.

we had this check already, but forgot to pick #233 to release-4.0

[overvenus]

LGTM

Please also add a test in another PR.