Here's our new watch strategy on Darwin in a nutshell:
1. Create an fsevents stream for events "since" the last event ID that
we saw globally.
2. Add a path that we want to watch
3. Add that path to a map of paths that we're watching _directly_.
4. Restart the event stream to pick up the new path.
5. Ignore all events for all watches until we've seen a `HistoryDone`
event.
6. Ignore the first `ItemCreated` event for paths we're watching that
are also directories
7. Otherwise, forward along all events.