Fix an RRULE bug with day cursors and skipped months

Summary:
Ref T10747. This rule wasn't quite correct, and got some things like BYMONTH=1,3;INTERVAL=9 wrong.

Instead of resetting the day cursor to 1, we need to move it forward through time in INTERVAL jumps until we get to the next candidate month.

Test Plan: Unit tests now pass.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16619