Query due tasks directly when data store isn’t yet synced

[andymatuschak]

It can take a few seconds (or more) to fully sync a data store onto a new client. To ensure that review sessions load quickly, we should first hit an API for due tasks when we haven't yet synced on a client. Review events created in that session will just be directly put to the API endpoint, so they won't be as durable, but I think that's fine—it's temporary.

[andymatuschak]

Will need #238 first.