backup: set concurrency to 1 when ratelimit enabled (#1015)

[ti-srebot]

cherry-pick #1015 to release-4.0
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/1025

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

git push git@github.com:ti-srebot/br.git pr/1025:release-4.0-0576f0713cd8

---

What problem does this PR solve?

Fix? #1007

What is changed and how it works?

When --ratelimit was set, degenerate to sequential backing up.

Check List

Tests

• Manual test (add detailed scripts or steps below)
  (See backup: --ratelimit doesn't work as intended #1007)

Release Note

• Fix a bug caused the real backup speed may go beyond far the --ratelimit.

[3pointer]

/run-all-tests

[kennytm]

/lgtm

[3pointer]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• 3pointer
• kennytm

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[3pointer]

/merge

[ti-chi-bot]

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

Details

Commit hash: 1358222

[kennytm]

/run-integration-test

CI network issues. https://internal.pingcap.net/idc-jenkins/blue/organizations/jenkins/br_ghpr_unit_and_integration_test/detail/br_ghpr_unit_and_integration_test/11518/pipeline

[ti-chi-bot]

@ti-srebot: Your PR was out of date, I have automatically updated it for you.

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