patch submission: truncate trailing nulls from heap rows to reduce the size of the null bitmap

The following patch truncates trailing null attributes from heap rows to reduce the size of the row bitmap. 

Applications often have wide rows in which many of the trailing column values are null. On an insert/update, all of the trailing null columns are tracked in the row bitmap. This can add a substantial overhead for very wide rows. This change truncates heap rows such that the trailing nulls are elided. 

The intuition for this change is that ALTER TABLE t ADD COLUMN c type NULL is a metadata only change. Postgres works fine when a row's metadata (tuple descriptor) is inconsistent with the actual row data: extra columns are assumed to be null. This change just adjusts the number of attributes for a row and the row bitmap to only track up to the last non-null attribute.

Thanks.

-Jamie Martin