kv-client: add stream binding background region worker, not enabled by default (#1397)

[ti-srebot]

cherry-pick #1397 to release-4.0
You can switch your code base to this Pull Request by using git-extras:

# In ticdc repo:
git pr https://github.com/pingcap/ticdc/pull/1455

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/ticdc.git pr/1455:release-4.0-5c4503cf90e9

---

What problem does this PR solve?

Preparatory work for #1393

What is changed and how it works?

1. Add a new configuration item in changefeed config, kv-client-v2, default false now, which will help us test the new feature. After refactor is stable, this config can be removed. (NOTE: add or remove a new item in changefeed config is both forward compatible and backward compatible) Use a hard code const variable kvClientV2, which is simpler
2. Keep original single routine event feed processing logic in cdc/kv/client.go
3. Add a new region worker implement in cdc/kv/region_worker.go, the region worker is single goroutine now, and has a state lock for all regions that it manages. One gRPC stream <-> One region worker (multiple routines support in another pr) <-> One resolve lock routine
4. Separate lock resolve logic from single region logic

NOTE:

• New logic keeps the same processing logic as much as possible against the each region one routine way, the unit test cases can be reused. I have tested that the client v2 can pass all integration tests and unit tests.
• Both step-3 and step-4 will be optimizated in next PR, this PR is reviewable and can be merged first.

Check List

Tests

• Unit test
• Integration test

Release note

• No release note

[ti-srebot]

/run-all-tests

[amyangfei]

I will resolve conflicts later

[amyangfei]

/run-all-tests

[amyangfei]

/run-integration-tests

[amyangfei]

/merge

[ti-srebot]

/run-all-tests

[codecov-io]

Codecov Report

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