ticdc/kvclient: make multiple cdc kv clients share one RegionCache (#3464)

[ti-chi-bot]

This is an automated cherry-pick of #3464

What problem does this PR solve?

issue 3208

Considering the following test:

• create a TiCDC cluster with one TiCDC server.
• create a changefeed using MySQL as TiCDC's sink target.
• create 10 tables and write some data to TiDB using sysbench prepare command.

Optimization Result:
Before optimization: In go pprof goroutine result, we can see 10 goroutines associated with *RegionCache).asyncCheckAndResolveLoop.
After optimization: In go pprof goroutine result, we can see 1 goroutines associated with *RegionCache).asyncCheckAndResolveLoop.

Note: Several goroutines created by tikv/client-go may be visible to us in go pprof goroutine result as well.

What is changed and how it works?

Use a server-level RegionCache instead of calling tikv.NewRegionCache every time a CDCClient is created. By this way, we can use exactly one goroutine for (*RegionCache).asyncCheckAndResolveLoop.

Check List

Tests

• Unit test
• Integration test

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has not been approved.

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[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.

To merge this cherry pick, it must first be approved by the collaborators.

AFTER it has been approved by collaborators, please ping the release team in a comment to request a cherry pick review.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.