R1:8efdc4aabf09

Replace getPagingValue() with cursor methods

Summary:
Ref T7803. Prior to this change sequence, Query classes conflated paging values (the actual thing that goes in a "x > 3" clause) with cursor values (arbitrary identifiers which track where the user is in a result list).

Although the two can sometimes be the same, the vast majority of implementations are simpler and better when object IDs are used as cursors and paging values are derived from them.

The new stuff handles this in a consistent way, so…