kv/client: add incremental scan region count limit (#1899)

[ti-chi-bot]

This is an automated cherry-pick of #1899

What problem does this PR solve?

This PR adds a token limiter to kv client, in order to control the concurrency of incremental scan region, for some reasons:

• A large scale incremental scan tasks could cause pressure in TiKV server, which could lead to OOM in TiKV.
• TiKV could also add concurrency limit of incremental scan regions and prevents too much memory consumption, however it will cause too many uninitialized pending regions in TiCDC kv clients. Region reconnect could happen before the TiKV can serve them.

What is changed and how it works?

• Add a token based region router.
  • When the used token doesn't reach upper limit, redirect the region request as soon as possible.
  • Otherwise buffer the region request, and check 100ms to see whether there exists new token opportunity.
  • The token is grouped by store address
• Separate some logic from dispatchRequest into requestRegionToStore
  • dispatchRequest only reads singleRegionInfo from region channel, try to get gRPC context and sends the singleRegionInfo(which has been filled in with gRPC context) into region router
  • requestRegionToStore reads singleRegionInfo from the output channel of region based region router, and sends real gRPC request to TiKV store.

Check List

Tests

• Unit test
• Integration test

Related changes

• Need to cherry-pick to the release branch
• Need to update key monitor metrics in both TiCDC document and official document

Release note

• Add concurrency limit to the region incremental scan in kv client.

[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