puller: ignore fallen back resolved ts

[amyangfei]

What problem does this PR solve?

This is an addition fix after #2038

In #2038, kv client tries to send resolved ts to puller after a region information is loaded from PD and dispatched in kv client. However, in extreme scenario, BatchLoadRegionsWithKeyRange could still miss some regions of intermediate states.

What is changed and how it works?

In the #2038 scenario, the fallen back resolved doesn't make any sense, because the resovled-ts in puller only guarantees no late data, but doesn't guarantee no repeated data. We have the same behavior in kv client when processing the resovled ts fallen back from TiKV, ref: https://github.com/pingcap/ticdc/blob/43c40f9d368a5f85beaf05080c3f11135648faa5/cdc/kv/client.go#L1473-L1480

• late data: It is the first time that a row changed event is received, and its commit-ts is smaller than resolved-ts.
• repeated data: It is not the first time that a row changed event is received, and its commit-ts is smaller than resolved-ts.

TODO, we should add more tests in span frontier module, and make sure the correctness. Some possible tasks:

• Support more fine-grained region forward strategy, to avoid non-essential resolved ts fallen back
• Add more robust and correctness tests for span frontier module

Check List

Tests

• Unit test
• Integration test

Release note

• No release note

[overvenus]

LGTM

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• liuzix
• overvenus

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[amyangfei]

/merge

[ti-chi-bot]

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

Commit hash: d893599

[codecov-commenter]

Codecov Report

Merging #2077 (3f6453e) into master (0138edc) will increase coverage by 16.0705%.
The diff coverage is 60.8783%.

@@                Coverage Diff                @@
##             master      #2077         +/-   ##
=================================================
+ Coverage   37.8006%   53.8711%   +16.0705%     
=================================================
  Files           110        164         +54     
  Lines         11394      17527       +6133     
=================================================
+ Hits           4307       9442       +5135     
- Misses         6662       7080        +418     
- Partials        425       1005        +580     

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #2086.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #2087.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #2088.