Revert "Capture video in a background queue. See: https://github.com/TheLevelUp/ZXingObjC/pull/74. This time I'm ensuring that the delegate methods are called in the main queue, so that the change goes well"

This reverts commit fe16eb4192a198a763fd41f367623e0dad674e99.
Messing with queues in ZXingObjC leads to all kinds of strange problems.