cdc: remove downstreams correctly when uninitialized

[hicqu]

Signed-off-by: qupeng [email protected]

What is changed and how it works?

#11896 (comment) introduces a critical bug for CDC, which can cause data lost or downstreams blocking forever. Here is how to trigger the bug:

1. multiple downstreams subscribe one region
2. cancel one downstream before the region delegate has been initialized

Then, Delegate::stop_observing will be called incorrectly and all change events will be lost.

This PR does

1. correct the bug
2. add a log for stop_observing
3. complete unit tests for Delegate::subscribe and Delegate::unsubscribe.

cdc: remove downstreams correctly when uninitialized

Related changes

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

Check List

Tests

• Unit test

Side effects

Release note

None

[ti-chi-bot]

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[hicqu]

/test

[overvenus]

Rest LGTM

[ti-chi-bot]

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[hicqu]

/merge

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[ti-chi-bot]

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

Commit hash: 455f259

[overvenus]

/merge

[ti-srebot]

cherry pick to release-6.0 in PR #12268