Re: Table AM modifications to accept column projection lists

This patch introduces a set of changes to the table AM APIs, making them
accept a column projection list. That helps columnar table AMs, so that
they don't need to fetch all columns from disk, but only the ones
actually needed.

The set of changes in this patch is not exhaustive -
there are many more opportunities that are discussed in the TODO section
below. Before digging deeper, we want to elicit early feedback on the
API changes and the column extraction logic.

TableAM APIs that have been modified are:

1. Sequential scan APIs
2. Index scan APIs
3. API to lock and return a row
4. API to fetch a single row

We have seen performance benefits in Zedstore for many of the optimized
operations [0]. This patch is extracted from the larger patch shared in
[0].