[close #395] Batch writes in Kafka consumer

[pingyu]

What problem does this PR solve?

Issue Number: close #395

Problem Description: cdc: Batch writes to sinks in Kafka consumer

What is changed and how does it work?

• Batch writes to sinks in Kafka consumer.
• Fix possible bugs:
  • MarkMessage should not be inside the loop of handling kv entries of a message.
  • break ClaimMessages when meet fallback kv seems to make no sense.

Depends on #392

Code changes

• No.

Check List for Tests

This PR has been tested by at least one of the following methods:

• Integration test

Side effects

• No side effects

Related changes

• No related changes

To reviewers

Please follow these principles to check this pull requests:

• Concentration. One pull request should only do one thing. No matter how small it is, the change does exactly one thing and gets it right. Don't mix other changes into it.
• Tests. A pull request should be test covered, whether the tests are unit tests, integration tests, or end-to-end tests. Tests should be sufficient, correct and don't slow down the CI pipeline largely.
• Functionality. The pull request should implement what the author intends to do and fit well in the existing code base, resolve a real problem for TiDB/TiKV users. To get the author's intention and the pull request design, you could follow the discussions in the corresponding GitHub issue or internal.tidb.io topic.
• Style. Code in the pull request should follow common programming style. (For Go, we have style checkers in CI). However, sometimes the existing code is inconsistent with the style guide, you should maintain consistency with the existing code or file a new issue to fix the existing code style first.
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(The above text mainly refers to TiDB Development Guide. It's also highly recommended to read about Writing code review comments)

[pingyu]

/test pull-integration-kafka-test

[pingyu]

/test pull-integration-kafka-test

[pingyu]

/test pull-integration-kafka-test

[pingyu]

/test pull-integration-kafka-test

[zeminzhou]

LGTM