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

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 writing /lgtm in a comment.
Reviewer can cancel approval by writing /lgtm cancel in a comment.

[3pointer]

/merge

[ti-chi-bot]

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

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@ti-srebot: Your PR was out of date, I have automatically updated it for you.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.