etcd_worker: batch etcd patch (#3277)

[ti-chi-bot]

This is an automated cherry-pick of #3277
This PR also include: https://github.com/pingcap/ticdc/pull/3681

What problem does this PR solve?

This PR is an improvement to the issue's(#3112 ) fix.

What is changed and how it works?

1. Batch as many changefeed patches as possible into a batch and submit them as an etcd transaction, which can reduce the etcd qps.
   2. Get the response from etcd watchCh as much as possible to reduce the number of etcd worker tick reactor.
2. Add metrics for some key points in EtcdWorker.

Check List

Tests

• Unit test

Side effects

• Possible performance regression
• Increased code complexity

Related changes

• Need to cherry-pick to the release branch

Release note

1. Fix the issue that TiCDC commits a txn contained too many operations to etcd which will caused txn commit fails.
2. Reduce the number of txn commit requests initiated by cdc to etcd which reducing the pressure of etcd server and network communication overhead.

[ti-chi-bot]

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[asddongmen]

/run-unit-tests

[asddongmen]

/run-integration-tests

[asddongmen]

/run-integration-tests

[codecov-commenter]

Codecov Report

Merging #3390 (8588ac6) into release-4.0 (753073a) will increase coverage by 3.2791%.
The diff coverage is 67.5675%.

@@                 Coverage Diff                 @@
##           release-4.0      #3390        +/-   ##
===================================================
+ Coverage      53.7661%   57.0452%   +3.2791%     
===================================================
  Files              153        170        +17     
  Lines            15958      19758      +3800     
===================================================
+ Hits              8580      11271      +2691     
- Misses            6475       7427       +952     
- Partials           903       1060       +157     

[amyangfei]

/merge

[ti-chi-bot]

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

Commit hash: 8588ac6