Sync data and index notes via Web Workers

[rtfeldman]

The following can and presumably should be done using one or more Web Workers:

1. IndexedDB persistence
2. Updating the Notes index
3. Syncing to Dropbox

It's unclear whether there would be any significant gains from giving each of these their own worker (as opposed to putting them all on the same worker), but syncing and updating the notes index are heavyweight enough operations that they definitely seem worth offloading to a different thread.