executor: Fix data race when getting snapshot ts in IndexLookupJoin and UnionExec (#30487)

[ti-srebot]

cherry-pick #30487 to release-5.4
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/31351

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---

What problem does this PR solve?

Issue Number: close #30468

Problem Summary: When IndexLookUpJoin builds its inner executors, the building procedure may try to activate the transaction to get the snapshot ts concurrently, causing data race. If we activate the transaction before building inner executors, this problem can be avoided.

It's a quick fix and it's not very elegant since concurrent accessing to the transaction state still exists. I think we'd better to refactor the whole architecture of these executors and the transaction state, however it would be a tough work.

What is changed and how it works?

• Revert executor: remove a dirty hack code which is useless now #20388 , so that the transaction will be activated before building the inner executors.
• Add some failpoints so that it can be tested in external integration tests.
• Check snapshot ts in buildUnionAll to avoid another similar data race in different threads of UnionExec (see Data race when sub-executors of IndexLookUpJoin gets snapshot ts #30468 (comment) )

Check List

Tests

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

Side effects

• None

Documentation

• None

Release note

Fix a data race that may cause "invalid transaction" error when executing a query using index lookup join.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• cfzjywxk
• tiancaiamao

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[ti-srebot]

/run-all-tests

[ti-srebot]

@MyonKeminta you're already a collaborator in bot's repo.

[tiancaiamao]

/merge

[ti-chi-bot]

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

Commit hash: ab9703f

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Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/51a1c02ca2c84f9413923034e9456be7f4953b0a