Mental note: Make API operations async

[karalabe]

Most API ops only touch the local database, remote sync is independent and happens in the background. This is good, we want eventual consistency. There are a few API calls that may also be pushed into the background (gateway startup, pairing startup). Although it's simpler to block, I agree that it can be problematic on the UI side if it has to wait indefinitely. Better to return fast if things look good, and provide a way to poll subsequently for eventual failures. It avoids having to mess with timeouts on the UI.