Fix an unusual internal cursor in Conpherence

Summary:
See https://discourse.phabricator-community.org/t/error-when-sending-a-message-chat-room/2548.

Conpherence calls setAfterID() and setBeforeID() directly on a subquery, but these methods no longer exist.

Use a pager instead. This code probably shouldn't exist (we should use some other approach to fetch this data in most cases) but that's a larger change.

Test Plan: Sent messages in a Conpherence thread. Before: fatal; after: success. Viewed the Conphrence menu, loaded threads, etc.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20318