R1:3f4cc3ad6eb4

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…