[SQL] Faster Scala row conversion

[liancheng]

This is a follow-up of #5279 and #5398. ScalaReflection.convertRowToScala is on a critical path, but was implemented in a rather inefficient way.

[SparkQA]

Test build #29856 has started for PR 5419 at commit d698d03.

[SparkQA]

Test build #29856 has finished for PR 5419 at commit d698d03.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.
• This patch does not change any dependencies.

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[marmbrus]

Do you mind reverting this to avoid conflicting with #5279?

[liancheng]

Actually the convertRowToScala part is already handled in #5279, and the lazy val schema part is handled in #5398, so I'm closing this.