types: match MySQL behavior with datetime delimiters (#17376)

[sre-bot]

cherry-pick #17376 to release-3.1

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What problem does this PR solve?

Issue Number: close #17277

Problem Summary: select date '2020-02--1' fails

What is changed and how it works?

• types.ParseDateFormat changed datetime splitting to match MySQL 5.7.
• Any ASCII punctuation character (i.e. printable non-alphanumeric) is a valid delimiter.
• Additionally, (space) and T are valid delimiters if they appear between time and date.
• Multiple consecutive delimiters are accepted, and handled as a single delimiter.
• Consecutive delimiters can be mixed (e.g. -./ or T/TT).
• This PR does not apply to types.ParseDuration, since it has additional validation.

Check List

• Unit tests

Backwards compatibility

• In previous TiDB versions, T and (space) were valid delimiters anywhere in a datetime string. After this change, using T or (space) as a delimiter is an error except between date and time.

Release note

• Datetime parsing now matches MySQL 5.7 behavior for delimiters.

[sre-bot]

/run-all-tests

[wjhuang2016]

@smola Could you resolve the conflict?

[Reminiscent]

@smola @wjhuang2016 PTAL

[wjhuang2016]

I don't think we should remove these benchmarks?

[Reminiscent]

@wjhuang2016 Update. PTAL

[wjhuang2016]

LGTM