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

[ti-srebot]

cherry-pick #12251 to release-3.1
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/12290

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

git push git@github.com:ti-srebot/tikv.git pr/12290:release-3.1-3364702e1903

---

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]

[REVIEW NOTIFICATION]

This pull request has not been approved.

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[ti-srebot]

/run-all-tests

[ti-srebot]

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

[NingLin-P]

/test