This patch is the first patch in UniqueKey patch series[1], since I need to revise
that series many times but the first one would be not that often, so I'd like to submit this one for review first so that I don't need to maintain it again and again.
Introduce notnullattrs field in RelOptInfo to indicate which attr are not null in current query. The not null is judged by checking pg_attribute and query's restrictinfo. The info is only maintained at Base RelOptInfo and Partition's RelOptInfo level so far.
