[C++] Improve UTF8 validation speed and CSV string conversion

[asfimport]

Based on profiling from ARROW-10308, UTF8 validation is a bottleneck of CSV string conversion.

This is because we must validate many small UTF8 strings individually.

Reporter: Antoine Pitrou / @pitrou
Assignee: Antoine Pitrou / @pitrou

Related issues:

• [Python] read_csv from python is slow on some work loads (relates to)

PRs and other links:

• GitHub Pull Request #8470

_{Note: This issue was originally created as ARROW-10313. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 8470
#8470