Replace MapBasedRow with ArrayBasedRow for groupBy

MapBasedRow is a row backed by a hash map which can incur substantial overheads while adding and getting items. This can become more significant as the result size grows.

Since the column order in the result is decided and fixed when a query is submitted, we can use ArrayBasedRow instead which is backed by an array.

Start by locating the groupBy result-row construction and reading the existing MapBasedRow and ArrayBasedRow implementations. Confirm the fixed result-column order is preserved, then verify that groupBy uses array-backed rows without changing result behavior.

Written by the indexing model from the issue text.