Add a "daily routine" trigger clock for backups, etc.

Summary: Ref T6881. Before implementing subscriptions, I'm going to vet triggers by using them to do backups. Each instance will get a daily trigger for backups, and that should give us a smaller-scale test to catch issues and limitations, with more opportunities for something to go wrong since it fires more often.

Test Plan: Added unit tests.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6881

Differential Revision: https://secure.phabricator.com/D11427