processor: cancel all tables all at once (#2089)

[ti-chi-bot]

This is an automated cherry-pick of #2089

What problem does this PR solve?

• The processor cancels tables one by one, meaning that the next table is not cancelled until the first table has fully exited. This causes risks of deadlocking.

What is changed and how it works?

• Cancel them all at once.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

Related changes

• Need to cherry-pick to the release branch

Release note

• No release note

[ti-chi-bot]

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[ti-chi-bot]

@ti-chi-bot: PR needs rebase.

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[amyangfei]

picked in https://github.com/pingcap/ticdc/pull/2142