Correctly handle initial sync

[gferon]

Right now, we rely on the client doing the contacts sync, running a message loop and handling all of the internals until ready to send messages. This isn't really nice (and easy) to use, and also incomplete.

Some notes:

• Store some flag in our store impl like SD does (something like [isInitialSync, bool]). Internally run a message loop on linking/registration
• Add missing pieces of processing contacts sync for the first time (for example, expiration timers - https://github.com/signalapp/Signal-Desktop/blob/3f0536f5a58b6a20949690afd76093f41931843c/ts/services/contactSync.ts#L79)
• Request more sync messages from primary device when linking as secondary device (configuration, keys)
• Figure out how to provision the list of groups on linking