Optimize fetching pages of results from the backend by using
cursor-based paging (via `search_after`) rather than offset-based paging
(via `offset`, `limit`). Cursor-based paging is more efficient to handle in
the backend as the starting position of a page within the overall result set increases.

This also fixes a race condition where results at the boundary between
pages might be fetched twice or not fetched if new matches for the query
become available while results are fetched.