Clean up some bits of UIImageView+AFNetworking.m

The largest change was setting the underlying dispatch queue to not be
the main thread, which should help make the UI feel buttery smooth.

Additionally:
1. -[NSMutableDictionary setValue:forKey:] can handle the value being
nil unlike -setObject...
2. The static const char* keys seem needlessly verbose considering
these are internal to the class(es) anyway
3. I moved [[UIImageView af_sharedImageCache] cacheImage:response
forKey:key] to the single standard completion block instead of that
call being duplicated multiple times (and thus called multiple times)
4. Lines 198/200/204 make use of the weakSelf/strongSelf semantic so
that the UIImageView can safely dealloc when appropriate