Fix storage configuration compatibility.

[JinheLin]

What problem does this PR solve?

Issue Number: close #2781, close #1760

Problem Summary:

What is changed and how it works?

What's Changed:

1. Remove useless configuration: storage.bg_task_io_rate_limit.
2. Move format_version and lazily_init_store from TiFlashStorageConfig::parse to TiFlashStorageConfig::parseMisc, and call parseMisc when configuration has [storage] section.
3. Rename TiFlashStorageConfig::parse to TiFlashStorageConfig::parseStoragePath and call parseStoragePath when configuration has [storage.main] section instead of has [storage] section.

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch:

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

Release note

none

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• JaySon-Huang

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 submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[JaySon-Huang]

Maybe we should totally remove bg_task_io_rate_limit from our docs (for the master and 5.2).
Since it is only an experimental config in v5.0/v5.1 and we don't support it anymore since v5.2

https://github.com/pingcap/docs-cn/blob/807a818b7f9c8fca4746509856ffdd5bfd95f984/tiflash/tiflash-configuration.md

[JaySon-Huang]

LGTM

[JinheLin]

/merge

[ti-chi-bot]

@JinheLin: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

You only need to trigger /merge once, and if the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

If you have any questions about the PR merge process, please refer to pr process.

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.

[ti-chi-bot]

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

Commit hash: 2f8938a

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #3049.