Summary: Ref T10537. Some sources (like the future "GitHub Repository" source) need to poll remotes. - Provide a mechanism for sources to emit import cursors. - Hook them into the trigger daemon so they'll fire periodically. - Provide some storage. This diff does nothing useful or interesting, and is pure infrastructure. Test Plan: - Ran `bin/storage upgrade -f`, no adjustment issues. - Poked around Nuance. - Ran the trigger daemon, verified it didn't crash and checked for Nuance stuff to do. Reviewers: chad Reviewed By: chad Maniphest Tasks: T10537 Differential Revision: https://secure.phabricator.com/D15435