*: avoid create new parser object in prepared exec

[tiancaiamao]

What problem does this PR solve?

Problem Summary:

PrepareExec.Next function checks whether we can use session's parser for parsing, otherwise a new parser object is allocated.

	if sqlParser, ok := e.ctx.(sqlexec.SQLParser); ok {
		// FIXME: ok... yet another parse API, may need some api interface clean.
		stmts, err = sqlParser.ParseSQL(e.sqlText, charset, collation)
	} else {
		p := parser.New()
               ...

However, we does not defined the correct interface method, so a new parser object is always allocated.
That would cause a lot of pressure to the Go garbage recycler.

What is changed and how it works?

What's Changed:

Fix the interface to avoid the parser allocation.

How it Works:

Related changes

• Need to cherry-pick to the release branch

Check List

Tests

• No code

This is enough, the check is done by the compiler:

var _ sqlexec.SQLParser = &session{}

Release note

• Optimize the parser object allocation in the prepared statement.

[XuHuaiyu]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• XuHuaiyu
• xhebox

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[tiancaiamao]

/merge

[ti-chi-bot]

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

Commit hash: b37e007

[ngaut]

/merge

[ti-chi-bot]

@ngaut: /merge is only allowed for the committers in list.

In response to this:

/merge

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[xhebox]

/run-unit-test

[tiancaiamao]

/run-all-tests

[tiancaiamao]

/run-unit-tes

[ti-chi-bot]

@tiancaiamao: Your PR was out of date, I have automatically updated it for you.

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