raftstore: only persist merge target if the merge is known to be succeeded (#12251)

[ti-srebot]

cherry-pick #12251 to release-3.0
You can switch your code base to this Pull Request by using git-extras:

# In tikv repo:
git pr https://github.com/tikv/tikv/pull/12292

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

git push git@github.com:ti-srebot/tikv.git pr/12292:release-3.0-3364702e1903

---

What is changed and how it works?

Issue Number: Close #12232

What's Changed:

When destroying, only persist merge target if the merge is known to be succeeded

raftstore: only persist merge target if the merge is known to be succeeded

Check List

Tests

• Integration test

Release note

fix tikv panic and peer unexpected destroy due to fake merge target

[ti-chi-bot]

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[ti-srebot]

/run-all-tests

[ti-srebot]

@NingLin-P you're already a collaborator in bot's repo.

[VelocityLight]

This pull request is closed because it's related version has closed automatic cherry-picking.
If it's still needed, you can reopen it or just regenerate it using bot,
see:

• https://prow.tidb.io/command-help#cherrypick
• https://book.prow.tidb.net/#/plugins/cherrypicker

You can find more details at:

• https://internals.tidb.io/t/topic/785