*: fix sysvar inconsistencies/potential bugs

[morgo]

What problem does this PR solve?

Problem Summary:

This fixes two inconsistencies/potential bugs with sysvars:

1. There are a number of Int/Unsigned sysvars that have a range which includes a MaxValue greater than the internal value which stores the setting, for example:

	{Scope: ScopeGlobal | ScopeSession, Name: TiDBExecutorConcurrency, Value: strconv.Itoa(DefExecutorConcurrency), Type: TypeUnsigned, MinValue: 1, MaxValue: math.MaxUint64, SetSession: func(s *SessionVars, val string) error {	{Scope: ScopeGlobal | ScopeSession, Name: TiDBExecutorConcurrency, Value: strconv.Itoa(DefExecutorConcurrency), Type: TypeUnsigned, MinValue: 1, MaxValue: math.MaxInt32, SetSession: func(s *SessionVars, val string) error {
		s.ExecutorConcurrency = tidbOptPositiveInt32(val, DefExecutorConcurrency)
		return nil
	}},

Math.Uint64 is much greater than tidbOptPositiveInt32. The precision is lost when setting the value, not in the validation function.. which makes this behavior undefined.

2. There are a number of variables which are inconsistently named using "TIDB" (all upper), when the convention is "TiDB". Since this is just the constant name, and doesn't change the storage name, it's safe to rename them.

What is changed and how it works?

Minor code fixes.

Related changes

-None

Check List

Tests

• Unit test (covered by existing tests)

Side effects

-None

Release note

• No release note

[morgo]

/run-check_dev_2

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• wjhuang2016
• xhebox

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[xhebox]

/merge

[ti-chi-bot]

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

Commit hash: 0664c0f

[morgo]

/run-check_dev_2

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