Support BYSETPOS for RRULE events with FREQ=HOURLY

Summary: Ref T10747. Most of these tests already passed; implement BYSETPOS logic for FREQ=HOURLY.

Test Plan: Unit tests now pass.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16629