store/tikv: make pub vars related to region to private and add functions to set & get

[AndreMouche]

Signed-off-by: AndreMouche

What problem does this PR solve?

This PR makes some public vars which is related to the region to private, and add public functions to set and get. By this PR, we could move and rename the vars freely in the go-client package and do not need to update the tools which are using these vars, for example br and tidb.

Part of #22513

Check List

Tests

• Unit test

Release note

• No release note

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• andylokandy
• youjiali1995

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.

[AndreMouche]

/merge

[ti-chi-bot]

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

Commit hash: b90f670

[AndreMouche]

/run-all-tests

[ti-chi-bot]

@AndreMouche: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

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.

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.

[AndreMouche]

/run-check_dev_2

[AndreMouche]

/run-all-tests

[AndreMouche]

/run-all-tests