WIP: running tests. Code needs a bit of clean up.

This patch completes the vectorized decoding with the goal of passing the existing
tests. There is still more patches to support the rest of the format spec, even
just for flat schemas.

This patch adds a new flag to enable the vectorized decoding. Tests were updated
to try with both modes where applicable.

Once this is working well, we can remove the previous code path.

Author: Nong Li <nong@databricks.com>

Closes #11055 from nongli/spark-12992-2.