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 we're free to separate getPagingValue() from paging. The new method is essentially getResultCursor().

This also implements getPageCursors(), which allows queries to return directional cursors. The inability to do this was a practical limitation blocking the implementation of T7803.

Test Plan:

• Browsed a bunch of results and paged through queries.
• Grepped for removed methods.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7803

Differential Revision: https://secure.phabricator.com/D12383