*: only get master status once when query-status (#1619)

[ti-srebot]

cherry-pick #1619 to release-2.0
You can switch your code base to this Pull Request by using git-extras:

# In dm repo:
git pr https://github.com/pingcap/dm/pull/1630

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/dm.git pr/1630:release-2.0-defac4af9192

---

What problem does this PR solve?

originally we get master status in every sync and relay unit, which is redundant because a worker handles only one source

What is changed and how it works?

get master status at worker level

Check List

Tests

• Pass original tests

Code changes

• Has interface methods change

Related changes

• Need to cherry-pick to the release branch

[lance6716]

/lgtm

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

[lance6716]

/merge

[ti-chi-bot]

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

Commit hash: 487b65c