experimental fix for NSOutputStream related crashes

- Don't schedule the stream on the main thread's runloop as AFNetworking is already scheduling it on its internal thread's runloop
- Similarly, don't close the stream as AFNetworking is already doing it
- Don't execute the stream callback method on the main thread but let it run on its thread, which is AFNetworking's internal thread
- Don't close the event source stream reader if the stream's buffer size exceeds a given size