Make Calendar recurrence sets return the indexes of recurrence dates properly

Summary:
Ref T11816. Currently, if you specify a start time, you get back indexes starting from that start time (0, 1, ...).

This can cause some bugs elsewhere when querying for recurring event ranges.

Instead, return the indexes consistently, even if there is a start time.

Test Plan: Added unit tests, made them pass. Fixed an existing unit test.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11816

Differential Revision: https://secure.phabricator.com/D17644