When a queue overflows we place the queue back in the stack and try
again. Because it's at the top of the stack we take exactly the same
queue and keep on looping forever if the other thread is blocked for
some reason.
Instead, mark the queue as overflowed and only place it back in the
stack when we have flushed it.
This avoids a deadlock when the main-thread invokes on the data loop
and blocks and when the data loop invokes on the main-thread and
overflows the queue.