region_cache: filter peers on tombstone or dropped stores (#24726)

[ti-srebot]

cherry-pick #24726 to release-5.0
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/25838

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git push git@github.com:ti-srebot/tidb.git pr/25838:release-5.0-55d26c583230

---

Signed-off-by: youjiali1995 zlwgx1023@gmail.com

What problem does this PR solve?

Issue Number: close #24648

Problem Summary:

TiDB doesn't handle tombstone or dropped stores correctly. It may report an error if there is a peer on such a store.

What is changed and how it works?

What's Changed:

1. Add a tombstone state which means the store is a tombstone.
2. Filter peers on tombstone or dropped stores and add a backoffer to Region.init().
3. Fix data race between asyncCheckAndResolveLoop() and initResolve(). Now unresolved stores will only be resolved by initResolve().

How it Works:

Check List

Tests

• Unit test

Side effects

Release note

• No release note.

[ti-chi-bot]

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[ti-chi-bot]

@ti-srebot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

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[ti-srebot]

/run-all-tests

[ti-srebot]

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