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

[ti-srebot]

cherry-pick #12251 to release-5.1
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# In tikv repo:
git pr https://github.com/tikv/tikv/pull/12293

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git push git@github.com:ti-srebot/tikv.git pr/12293:release-5.1-3364702e1903

---

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 been approved by:

• BusyJay
• ekexium

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[ti-srebot]

/run-all-tests

[ti-srebot]

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