Run text extraction from images serially

[pjrobertson]

Avoids overwhelming resources when many images are being scanned (previously, it would fire of an unlimited number of background threads equal to the number of images being scanned).

Changes:

Replaced QSGCDAsync (global concurrent queue) with a dedicated DISPATCH_QUEUE_SERIAL queue at QOS_CLASS_UTILITY priority. This means image text recognition requests are processed one at a time on a low-priority background thread, instead of hundreds firing concurrently and starving the system.