processor: fix a bug that will cause processor Tick get stuck when downstream is Kafka (#11339)

[ti-chi-bot]

This is an automated cherry-pick of #11339

What problem does this PR solve?

Issue Number: close #11340

What is changed and how it works?

Check List

Tests

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

Deploy a cdc cluster, create two changefeeds and synchronize the same table to both Kafka and TiDB separately.

Every 20 minutes, inject CDC to network isolation:

• First, isolate cdc to Kafka network for 10 minutes.
• Then, 20 minutes later, isolate cdc to downstream TiDB network for 10 minutes.

Repeat this process six times.

Before this PR, the lag in the MySQL sink changefeed was influenced by the Kafka sink changefeed when network isolation was introduced between CDC and Kafka:

After this PR, only Kafka sink changefeed lag increasing during network isolation injection:

Questions

Will it cause performance regression or break compatibility?

Do you need to update user documentation, design documentation or monitoring documentation?

Release note

Fix a bug that causes the processor tick to get stuck when the downstream is Kafka and becomes unreachable.```

[ti-chi-bot]

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This pull-request has been approved by: asddongmen

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[asddongmen]

/test verify

[asddongmen]

/test verify