Fix TiFlash overflow (#2740)

[ti-chi-bot]

This is an automated cherry-pick of #2740

What problem does this PR solve?

Reproduce:

create and insert

CREATE TABLE test.t(`a` bigint(20) UNSIGNED NOT NULL)
insert into t values (16717361816800086255)

query from tispark with tiflash returns an error result（negative number）

 spark.conf
        .set("spark.tispark.isolation_read_engines","tiflash")
spark.sql("select * from tidb_catalog.test.t").show()

What is changed and how it works?

TiSpark requests TiFlash using TypeCHBlock encode, but forgets to handle the UNSIGNED scenes when decoded.

this pr shows how to decode it corretly

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation
• Need to update the tidb-ansible repository
• Need to be included in the release note

[shiyuhang0]

/run-all-tests

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@xuanyu66: adding LGTM is restricted to approvers and reviewers in OWNERS files.

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[shiyuhang0]

/merge

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[shiyuhang0]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: shiyuhang0, xuanyu66

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