ExecuteFetchAsDBA(): handle 'allowZeroInDate' and batched queries

[shlomi-noach]

Description

This is a backport-compliant fix for #14952. In this PR:

• ExecuteFetchAsDBA() correctly identifies the existence of /*vt+ allowZeroInDate=true */ in either a single or a multi statement SQL. However, this is a hacky solution in that it is enough that a single query has this directive, that we apply it to all queries.
• ExecuteFetchAsDBA() drains the results of a multi-statement SQL, and as effect ReloadSchema loads all tables effected in a multi-statement SQL.

This is backport compliant solution, and so it intentionally does not error when a multi-statement SQL is provided. Technically, it's not the correct mechanism for running multiple queries, because it only returns a single result set.

A followup PR, that will build on top of this one for backwards compatibility reasons, will make proto changes to better formalize multi-statement SQLs as well as the request for a allowZeroInDate friendly connection.

Backport

This should be backported to release-18.0 because --batch-size is included in release-18.0.

Related Issue(s)

• Bug Report: ApplySchema --batch-size fails to apply zero-date and causes premature ReloadSchema #14952
• ExecuteFetch misbehavior when running multiple queies #14948

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[shlomi-noach]

Making some edits, will re-publish shortly.

[shlomi-noach]

Replaced by #14954