*: consitent get infoschema

[xhebox]

What problem does this PR solve?

The main issue: #24233

Problem Summary: In some cases, TxnCtx.InfoSchema is directly used while there may be SnapshotInfoSchema. There was a similar fix #15258, only for distsql. There are more left cases:

executor/coprocessor.go: buildDAGExecutor
executor/infoschema_reader.go: setDataForTiKVRegionStatus, setDataForTiDBHotRegions
expression/builtin_info.go: builtinNextValSig.evalInt, builtinLastValSig.evalInt, builtinSetValSig.evalInt
planner/core/expression_rewriter.go: rewriteAstExpr
planner/core/integration_test.go: TestPartitionPruningForEQ

While I want to replace all TxnCtx.InfoSchema into infoschema.GetInfoSchema(sessionCtx), I found that there will be a circular dependency in expression -> ... -> infoschema -> expression.

So I moved GetInfoSchema function into sessionctx/variable, as a method of SessionVars. Then replaced all calls into sessionVars.GetInfoSchema().(infoschema.InfoSchema). For expression package, it is sessionVars.GetInfoSchema().(util.SequenceSchema). That is why this PR is large.

The call is a bit longer with the extra GetSessionVars() and type cast .(infoschema.InfoSchema), but it works and removed one of four sessionctx imports in the infoschema package. Hopefully infoschema will not depend on sessionctx in the future...

Check List

Tests

• Unit test
• Integration test

Release note

• Fix tidb_snapshot did not take effect in some cases

[AilinKid]

/run-all-tests

[xhebox]

/run-unit-test

[djshow832]

/lgtm

[bb7133]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• bb7133
• djshow832

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[bb7133]

/merge

[ti-chi-bot]

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

Commit hash: 02152ca

[ti-chi-bot]

/run-all-tests

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[ti-srebot]

cherry pick to release-4.0 in PR #24528

[ti-srebot]

cherry pick to release-5.0 in PR #24529