puller,mounter,processor: always pull the old value internally (#2271)

[ti-chi-bot]

This is an automated cherry-pick of #2271

What problem does this PR solve?

ref: https://github.com/pingcap/ticdc/issues/2301

What is changed and how it works?

ticdc always pull the old value internally.

Because our new collation feature needs the old value to work properly. However, some users will not use old value, but may use new collation, so it needs to be configured by the user. (Users don't even know if they have new collation on)

To simplify this configuration and to make it easier for the user, we decided to:

1. always use old value internally.
2. the data output to the user needs to be in the correct format (without the old value configuration on, the old value cannot be output)

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

Code changes

None

Side effects

• Possible performance regression

Related changes

• Need to cherry-pick to the release branch

Release note

• puller,mounter,processor: always pull the old value internally

[ti-chi-bot]

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[amyangfei]

Please fix CI, github.com/tikv/client-go/v2 is not available in all release branches, because https://github.com/pingcap/ticdc/pull/2109 is not cherry-picked. @hi-rustin

[Rustin170506]

I will fix it.

[amyangfei]

/merge

[ti-chi-bot]

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

Commit hash: 0953c64

[amyangfei]

/run-leak-tests
/run-integration-tests

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (release-5.1@a9d7bfb). Click here to learn what that means.
The diff coverage is n/a.

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[amyangfei]

/run-integration-tests