RFE: A way to know if/when at least one sync has happened

[aboodman]

People often want to know when the fist sync round trip has happened. One common reason for this is to initialize state with some default data. Waiting for onSync(true) sort of works except:

• if the client already has local state, this is needlessly slow
• it doesn't work at all in the offline case - onSync(true) will fire even if the first sync was unsuccesful, leading the app to incorrectly add the default data

The simplest way I can think of to accomplish this is to add a promise like firstServerResponse: Promise<void>. If the client knows it has already had one server response, it can resolve this immediately, otherwise it can wait.