backup: allow backup tolerate minor TiKV failure (#997)

[ti-srebot]

cherry-pick #997 to release-5.0
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/1019

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

git push [email protected]:ti-srebot/br.git pr/1019:release-5.0-db863d52a070

---

What problem does this PR solve?

fixed #980

What is changed and how it works?

ignore all ‘failed to connect to store’ errors during backup.

Check List

Tests

• Integration test

Release Note

• BR now can tolerate minor TiKV disconnection.

[YuJuncen]

/run-integration-tests

[YuJuncen]

/run-integration-tests

[YuJuncen]

/run-integration-tests

[3pointer]

/lgtm

[YuJuncen]

/run-integration-tests

[lichunzhu]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• 3pointer
• lichunzhu

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[3pointer]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: b0919cf

[ti-srebot]

cherry pick to release-4.0 in PR #1062