Fix a straggling issue with cursor changes impacting Conpherence thread indexing

Summary: Ref T13266. Caught one more of these "directly setting afterID" issues in the logs.

Test Plan: Ran bin/search index --type ConpherenceThread before and after changes. Before: fatal about a direct call. After: clean index rebuild.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13266

Differential Revision: https://secure.phabricator.com/D20341