Moving code away from the main thread

[BellAppLab]

FastttCamera now calls beginBackgroundTaskWithExpirationHandler upon initialising and calls endBackgroundTask upon tearing down.

The camera also enqueues blocks on its own serial NSOperationQueue rather than relying on the system's global queue (see #5).

[HePingLaoSan]

Should not lazy initialize serial queue, because it may cause data race