Fix an issue where an excessively long TTL was computed for "not attending anything" event caches

Summary:
Fixes T11894. Currently, if you aren't attending any events for a while, we can cache that you are free for the next 72 hours, even if you have an event in a few hours.

Instead, only cache "user is free" until the next event, if one exists.

Test Plan: Dumped cache TTLs, saw 52 minutes instead of ~4300 minutes with a near-upcoming event.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11894

Differential Revision: https://secure.phabricator.com/D16937