Add sync state API

[carlsverre]

The client app needs a way to query the current synchronisation state.

• Which databases are subscribed?
• How much of our timeline has been sent to the server?
• How far behind is the server from applying our changes?

Ideally this ties into the mutation API so the client can also use it to await a specific mutation. This would allow the developer to opt into synchronous request/response if needed.

[carlsverre]

The TODO demo should show this status in a nice way. Both to expose the concept more clearly and to provide a better TODO UX.