kv/client: add global grpc connection pool (#2511)

[ti-chi-bot]

This is an automated cherry-pick of #2511

What problem does this PR solve?

Add global gRPC connection pool in cdc server.

What is changed and how it works?

• The gRPC pool is shared by all kv clients in a capture, the pool is created before a capture starts running and destroyed after a capture suicides or exits.
• The gPRC pool maintains a bucket of gPRC connections for each TiKV store, the bucket is increased in a lazy way.
• The gPRC pool provides a builtin recycle method, which is used to collect idle connections periodically.

Check List

Tests

• Unit test
• Integration test

Release note

Add a global gRPC connection pool and share gRPC connections among kv clients.

[ti-chi-bot]

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[amyangfei]

/run-integration-tests
/run-kafka-tests

[codecov-commenter]

Codecov Report

Merging #2531 (5c634df) into release-4.0 (753073a) will increase coverage by 1.3210%.
The diff coverage is 58.3305%.

@@                 Coverage Diff                 @@
##           release-4.0      #2531        +/-   ##
===================================================
+ Coverage      53.7661%   55.0872%   +1.3210%     
===================================================
  Files              153        164        +11     
  Lines            15958      19667      +3709     
===================================================
+ Hits              8580      10834      +2254     
- Misses            6475       7804      +1329     
- Partials           903       1029       +126     

[3AceShowHand]

/run-integration-tests
/run-kafka-tests

[amyangfei]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 5c634df

[ti-chi-bot]

@ti-chi-bot: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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[amyangfei]

/run-leak-tests