Implement fast skiplist-based aggregation

This is a major enhancement and will take some time to develop.

Currently cstore just returns individual tuples and relies on PostgreSQL for aggegation. This means we're not getting one of the primary benefits of column stores, which is fast single-column aggregation, based on the skiplist indexes.

This would probably involve implementing an executor hook. It will be complicated to figure out how to use the hook when we can (i.e. single-column known aggregation) and to kick it out to Postgres when we can't (e.g. multicolumn aggregation, custom aggregates, non-btree aggregation, etc.). Ideally, we would also add an API for supporting custom aggregates which can be based on COUNT/MIN/MAX from the skiplists.