*: read downstream table before CREATE TABLE to fix IF NOT EXISTS (#1915)

[ti-chi-bot]

This is an automated cherry-pick of #1915

What problem does this PR solve?

revert some of #1699

What is changed and how it works?

as title, we read downstream table from checkpoint/etcd/SHOW CREATE TABLE first, to deal with IF NOT EXISTS.

in future we'll change to a better strategy.

Check List

Tests

• pass original tests

Code changes

Side effects

• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to be included in the release note

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• lance6716

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.

[lance6716]

/merge

[ti-chi-bot]

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

Commit hash: 4ca24b9