kv-client: use worker pool in region worker (#1481)

[ti-chi-bot]

This is an automated cherry-pick of #1481

What problem does this PR solve?

Part of #1393

To increase the throughput of kv client, we should use multiple goroutines in region worker

What is changed and how it works?

• Reuse github.com/pingcap/ticdc/pkg/workerpool

Check List

Tests

• Unit test
• Integration test

Release note

• No release note

[ti-chi-bot]

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[ti-chi-bot]

@ti-chi-bot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

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[ti-chi-bot]

@ti-chi-bot: PR needs rebase.

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[amyangfei]

Cherry-picked in #2149