Allow Nuances sources to provide import cursors

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