Remove all uses of dispatch_get_current_queue, except for logging.

All uses of dispatch_get_current_queue (deprecated since iOS 6.0) are removed,
and replaced by assigning a static queue tag, which is checked for its value.
Inside the queue, the specific will return itself, while outside the queue it
will return NULL, which will make the check fail.

Only two instances of dispatch_get_current_queue remain, both have been
surrounded by pragmas to disable the warning by the Clang compiler.
- DDLog.m: obtaining the current queue label.
- DelegateTesters.m: obtaining the current queue label for NSLog.