prepare: filter databases before show table status

[lichunzhu]

What problem does this PR solve?

In #305, we have switched the method to get table info to SHOW TABLE STATUS. To reduce the access to databases, we'd better filter databases before we executing SHOW TABLE STATUS.

What is changed and how it works?

Filter databases before SHOW TABLE STATUS through conf.Filter.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  Dump databases and check the log. The databases are filtered before SHOW TABLE STATUS.

Related changes

• Need to cherry-pick to the release branch

Release note

• No release note

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

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

[lichunzhu]

/merge

[ti-chi-bot]

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

Commit hash: f375384

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #336.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #337.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #338.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #367.